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1.  Redetermination of junitoite, CaZn2Si2O7·H2O 
The crystal structure of the mineral junitoite, ideally CaZn2Si2O7·H2O (calcium dizinc disilicate monohydrate), was first determined by Hamilton & Finney [Mineral. Mag. (1985), 49, 91–95] based on the space group Ama2, yielding a reliability factor R of 0.10, with isotropic displacement parameters for all non-H atoms. The present study reports a structure redetermination of junitoite using single-crystal X-ray diffraction data from a natural sample, demonstrating that the space group of this mineral is actually Aea2, which can be attained simply by shifting the origin. Topologically, the structure models in the space groups Aea2 and Ama2 are analogous, consisting of chains of corner-sharing ZnO4 tetra­hedra parallel to the b axis, cross-linked by Si2O7 tetra­hedral dimers (the site symmetry of the bridging O atom is ..2) along a and c, forming a three-dimensional framework. The Ca2+ cations (site symmetry ..2) are situated in cavities of the framework and are bonded to five O atoms and one H2O mol­ecule (site symmetry ..2) in a distorted octa­hedral coordination environment. However, some bond lengths, especially for the SiO4 tetra­hedron, are noticeably different between the two structure models. Hydrogen bonding in junitoite is found between the water mol­ecule and a framework O atom.
doi:10.1107/S1600536812037622
PMCID: PMC3470120  PMID: 23125564
2.  Nonapotassium trialuminium hexa­phosphate 
In the title compound, K9Al3(PO4)6, the anionic substructure is built of inter­linked [PO4] and [AlO4] tetra­hedra. Each O atom of the [AlO4] tetra­hedron is common to a positionally different [PO4] tetra­hedron; thus, each [AlO4] tetra­hedron is surrounded by four positionally different [PO4] tetra­hedra. On the other hand, each [PO4] tetra­hedron shares its two O atoms with two positionally different [AlO4] tetra­hedra; the other two phosphate O atoms are terminal ones coordinated by K atoms. The terminal O atoms are usually closer to the K atoms than the bridging O atoms between the [AlO4] and [PO4] tetra­hedra. There are nine symmetry-independent K atoms in the structure. The coordination numbers of the K atoms are 6 or 7 or 8 up to a distance of 3.31 Å. There are channels in the anionic substructure oriented along the [10] direction that are filled by K atoms.
doi:10.1107/S160053681001305X
PMCID: PMC2979140  PMID: 21578985
3.  Thortveitite-type Tm2Si2O7  
Single crystals of dithulium disilicate, Tm2Si2O7, were obtained in flux synthesis experiments in the system SiO2–Tm2O3–LiF at ambient pressure. The compound belongs to the group of sorosilicates, i.e. it is based on [Si2O7]-units and crystallizes in the thortveitite (Sc2Si2O7) structure type. The Tm3+ cation (site symmetry .2.) occupies a distorted octa­hedral site, with Tm—O bond lengths in the range 2.217 (4)–2.289 (4) Å. Each of the octa­hedra shares three of its edges with adjacent [TmO6] groups, resulting in the formation of layers parallel to (001). The individual [SiO4] tetra­hedra are more regular, i.e. the differences between the bond lengths between Si and the bridging and non-bridging O atoms are not very pronounced. The layers containing the octa­hedra and the sheets containing the [Si2O7] groups (point group symmetry 2/m) form an alternating sequence. Linkage is provided by sharing common oxygen vertices.
doi:10.1107/S1600536814013142
PMCID: PMC4120532  PMID: 25161507
4.  Crystal structure of [NiHg(SCN)4(CH3OH)2]n  
The crystal structure of [NiHg(SCN)4(CH3OH)2]n is made up of HgS4 tetra­hedra and trans-NiN4O2 octa­hedra, linked together by thio­cyanato bridges. The methanol mol­ecules point to the cavities of the resulting framework.
The title compound, catena-poly[[bis­(methanol-κO)nickel(II)]-di-μ-thio­cyanato-κ4 N:S-mercurate(II)-di-μ-thio­cyanato-κ4 N:S], was obtained from a gel-growth method using tetra­meth­oxy­silane as gelling agent. The crystal structure is composed of rather regular HgS4 tetra­hedra (point group symmetry .2.) and trans-NiN4O2 octa­hedra (point group symmetry 2..) that are linked through thio­cyanato bridges into a three-dimensional framework. The methanol mol­ecules coordinate via the O atom to the Ni2+ cations and point into the voids of this arrangement while a weak O—H⋯S hydrogen bond to an adjacent S atom stabilizes it.
doi:10.1107/S1600536814009532
PMCID: PMC4158493  PMID: 25249851
crystal structure; NLO materials; gel growth; nickel; mercury
5.  Redetermination of tamarugite, NaAl(SO4)2·6H2O 
The crystal structure of tamarugite [sodium aluminium bis­(sulfate) hexa­hydrate] was redetermined from a single crystal from Mina Alcaparossa, near Cerritos Bayos, southwest of Calama, Chile. In contrast to the previous work [Robinson & Fang (1969 ▶). Am. Mineral. 54, 19–30], all non-H atoms were refined with anisotropic displacement parameters and H-atoms were located by difference Fourier methods and refined from X-ray diffraction data. The structure is built up from nearly regular [Al(H2O)6]3+ octa­hedra and infinite double-stranded chains [Na(SO4)2]3− that extend parallel to [001]. The Na+ cation has a strongly distorted octa­hedral coordination by sulfate O atoms [Na—O = 2.2709 (11) – 2.5117 (12) Å], of which five are furnished by the chain-building sulfate group S2O4 and one by the non-bridging sulfate group S1O4. The [Na(SO4)2]3− chain features an unusual centrosymmetric group formed by two NaO6 octa­hedra and two S2O4 tetra­hedra sharing five adjacent edges, one between two NaO6 octa­hedra and two each between the resulting double octa­hedron and two S2O4 tetra­hedra. These groups are then linked into a double-stranded chain via corner-sharing between NaO6 octa­hedra and S2O4 tetra­hedra. The S1O4 group, attached to Na in the terminal position, completes the chains. The [Al(H2O)6]3+ octa­hedron (〈Al—O〉 = 1.885 (11) Å) donates 12 comparatively strong hydrogen bonds (O⋯O = 2.6665 (14) – 2.7971 (15) Å) to the sulfate O atoms of three neighbouring [Na(SO4)2]3− chains, helping to connect them in three dimensions, but with a prevalence parallel to (010), the cleavage plane of the mineral. Compared with the previous work on tamarugite, the bond precision of Al—O bond lengths as an example improved from 0.024 to 0.001 Å.
doi:10.1107/S1600536813025154
PMCID: PMC3790334  PMID: 24098156
6.  Sr–fresnoite determined from synchrotron X-ray powder diffraction data 
The fresnoite-type compound Sr2TiO(Si2O7), distrontium oxidotitanium disilicate, has been prepared by high-temperature solid-state synthesis. The results of a Rietveld refinement study, based on high-resolution synchrotron X-ray powder diffraction data, show that the title compound crystallizes in the space group P4bm and adopts the structure of other fresnoite-type mineral samples with general formula A2TiO(Si2O7) (A = alkaline earth metal cation). The structure consists of titanosilicate layers composed of corner-sharing SiO4 tetra­hedra (forming Si2O7 disilicate units) and TiO5 square-based pyramids. These layers extend parallel to the ab plane and are stacked along the c axis. Layers of distorted SrO6 octa­hedra lie between the titanosilicate layers. The Sr2+ ion, the SiO4 tetra­hedron and the bridging O atom of the disilicate unit are located on mirror planes whereas the TiO5 square-based pyramid is located on a fourfold rotation axis.
doi:10.1107/S1600536812048921
PMCID: PMC3588266  PMID: 23476310
7.  Disodium calcium dinickel(II) bis­[diphosphate(V)] deca­hydrate 
In the title compound, Na2CaNi2(P2O7)2(H2O)10, there are two distinct P-atom sites, each tetra­hedrally coordinated by four O atoms. The resulting phosphate tetra­hedra link through a common O atom, forming a [P2O7]4− diphosphate unit. The Ni—O coordination is square pyramidal with four O atoms from two diphosphate groups in equatorial positions and the vertex occupied by a water O atom. The (P2O7)(H2O) units link the Ni atoms, forming a chain of pyramids and tetra­hedra. As a result of the d-glide and twofold-axis symmetry of space group Fdd2, the chains propagate along [101] and [10], and chains in adjacent layers are mutually orthogonal. The Ca cation, located on a rotation axis, and the Na cation are each octa­hedrally coordinated by four O atoms and two waters. The Ni-chain arrangement is stabilized by Ca and Na coordination and a network of O—H⋯O hydrogen bonds.
doi:10.1107/S160053681100016X
PMCID: PMC3051721  PMID: 21522811
8.  Crystal structures of Sr(ClO4)2·3H2O, Sr(ClO4)2·4H2O and Sr(ClO4)2·9H2O 
The crystal structures of the tri-, tetra- and nona­hydrate phases of Sr(ClO4)2 consist of Sr2+ ions coordinated by nine oxygen atoms from water mol­ecules and perchlorate tetra­hedra. O—H⋯O hydrogen bonds between water mol­ecules and ClO4 units lead to the formation of a three-dimensional network in each of the structures.
The title compounds, strontium perchlorate trihydrate {di-μ-aqua-aquadi-μ-perchlorato-strontium, [Sr(ClO4)2(H2O)3]n}, strontium perchlorate tetra­hydrate {di-μ-aqua-bis­(tri­aqua­diperchloratostrontium), [Sr2(ClO4)4(H2O)8]} and strontium perchlorate nona­hydrate {hepta­aqua­diperchloratostrontium dihydrate, [Sr(ClO4)2(H2O)7]·2H2O}, were crystallized at low temperatures according to the solid–liquid phase diagram. The structures of the tri- and tetra­hydrate consist of Sr2+ cations coordinated by five water mol­ecules and four O atoms of four perchlorate tetra­hedra in a distorted tricapped trigonal–prismatic coordination mode. The asymmetric unit of the trihydrate contains two formula units. Two [SrO9] polyhedra in the trihydrate are connected by sharing water mol­ecules and thus forming chains parallel to [100]. In the tetra­hydrate, dimers of two [SrO9] polyhedra connected by two sharing water mol­ecules are formed. The structure of the nona­hydrate contains one Sr2+ cation coordinated by seven water mol­ecules and by two O atoms of two perchlorate tetra­hedra (point group symmetry ..m), forming a tricapped trigonal prism (point group symmetry m2m). The structure contains additional non-coordinating water mol­ecules, which are located on twofold rotation axes. O—H⋯O hydrogen bonds between the water mol­ecules as donor and ClO4 tetra­hedra and water mol­ecules as acceptor groups lead to the formation of a three-dimensional network in each of the three structures.
doi:10.1107/S1600536814024726
PMCID: PMC4257379  PMID: 25552979
crystal structure; low-temperature salt hydrates; perchlorate hydrates; strontium salts
9.  Redetermination of durangite, NaAl(AsO4)F 
The crystal structure of durangite, ideally NaAl(AsO4)F (chemical name sodium aluminium arsenate fluoride), has been determined previously [Kokkoros (1938). Z. Kristallogr. 99, 38–49] using Weissenberg film data without reporting displacement parameters of atoms or a reliability factor. This study reports the redetermination of the structure of durangite using single-crystal X-ray diffraction data from a natural sample with composition (Na0.95Li0.05)(Al0.91Fe3+ 0.07Mn3+ 0.02)(AsO4)(F0.73(OH)0.27) from the type locality, the Barranca mine, Coneto de Comonfort, Durango, Mexico. Durangite is isostructural with minerals of the titanite group in the space group C2/c. Its structure is characterized by kinked chains of corner-sharing AlO4F2 octa­hedra parallel to the c axis. These chains are cross-linked by isolated AsO4 tetra­hedra, forming a three-dimensional framework. The Na+ cation (site symmetry 2) occupies the inter­stitial sites and is coordinated by one F− and six O2− anions. The AlO4F2 octa­hedron has symmetry -1; it is flattened, with the Al—F bond length [1.8457 (4) Å] shorter than the Al—O bond lengths [1.8913 (8) and 1.9002 (9) Å]. Examination of the Raman spectra for arsenate minerals in the titanite group reveals that the position of the band originating from the As—O symmetric stretching vibrations shifts to lower wavenumbers from durangite, maxwellite [ideally NaFe(AsO4)F], to tilasite [CaMg(AsO4)F].
doi:10.1107/S160053681204384X
PMCID: PMC3515088  PMID: 23284315
10.  Trilithium thio­arsenate octa­hydrate 
The title compound, Li3AsS4·8H2O, is built up from infinite cationic [Li3(H2O)8]3+ chains which extend along [001] and are cross-linked by isolated tetra­hedral AsS4 3− anions via O—H⋯S hydrogen bonds. Two Li and two As atoms lie on special positions with site symmetries -1 (1 × Li) and 2 (1 × Li and 2 × As). The [Li3(H2O)8]3+ chain contains four independent Li atoms of which two are in octa­hedral and two in tetra­hedral coordination by water O atoms. An outstanding feature of this chain is a linear group of three edge-sharing LiO6 octa­hedra to both ends of which two LiO4 tetra­hedra are attached by face-sharing. Such groups of composition Li5O16 are linked into branched chains by means of a further LiO4 tetra­hedron sharing vertices with four adjacent LiO6 octa­hedra. The Li—O bonds range from 1.876 (5) to 2.054 (6) Å for the LiO4 tetra­hedra and from 2.026 (5) to 2.319 (5) Å for the LiO6 octa­hedra. The two independent AsS4 3− anions have As—S bond lengths ranging from 2.1482 (6) to 2.1677 (6) Å [ = 2.161 (10) Å]. The eight independent water mol­ecules of the structure donate 16 relatively straight O—H⋯S hydrogen bonds to all S atoms of the AsS4 tetra­hedra [ = 3.295 (92) Å]. Seven water mol­ecules are in distorted tetra­hedral coordination by two Li and two S; one water mol­ecule has a flat pyramidal coordination by one Li and two S. At variance with related compounds like Schlippe’s salt, Na3SbS4·9H2O, there are neither alkali–sulfur bonds nor O—H⋯O hydrogen bonds in the structure.
doi:10.1107/S1600536813010921
PMCID: PMC3647787  PMID: 23723753
11.  Disilver(I) trinickel(II) hydrogenphos­phate bis­(phosphate), Ag2Ni3(HPO4)(PO4)2  
The title compound, Ag2Ni3(HPO4)(PO4)2, has been synthesized by the hydro­thermal method. Its structure is formed by two types of chains running along the b axis. The first chain results from a linear and continuous succession of NiO6 octa­hedra linked to PO4 tetra­hedra by a common vertex. The second chain is built up from two adjacent edge-sharing octa­hedra (dimers) whose ends are linked to two PO4 tetra­hedra by a common edge. Those two types of chains are linked together by the phosphate groups to form polyhedral sheets parallel to the (001) plane. The three-dimensional framework delimits two types of hexa­gonal tunnels parallel to the a-axis direction, at (x, 1/2, 0) and (x, 0, 1/2), where the Ag atoms are located. Each silver cation is surrounded by eight O atoms. The same Ag+ coordination is found in other phosphates with the alluaudite structure, for example, AgMn3(PO4)(HPO4)2. Moreover, O—H⋯O hydrogen bonds link three PO4 tetra­hedra so as to build a three-dimensional network.
doi:10.1107/S1600536811021167
PMCID: PMC3151764  PMID: 21836825
12.  Mg6.75(OH)3(H0.166AsO4)3(HAsO4), a member of the M 1- xM′6(OH)3(H2x/3AsO4)3(HAsO4) family (M,M′ = Co; Ni) 
In the structure of the title compound, magnesium hydroxide hydrogenarsenate (6.75/3/4), two different Mg2+ ions, one located on a site with symmetry 3m. (occupancy 3/4) and one on a general position, as well as two different AsO3(OH) tetra­hedra (symmetry .m. with partial occupancy for the H atom for one, and symmetry 3m. with full occupancy for the H atom for the other) and one OH− ion (site symmetry .m.) are present. Both Mg2+ ions are octa­hedrally surrounded by O atoms. The MgO6 octa­hedra belonging to the partially occupied Mg2+ sites share faces, forming chains along [001]. The other type of MgO6 octa­hedra share corners and faces under formation of strands parallel to [001] whereby individual strands are linked through common corner atoms. The two types of AsO3(OH) tetra­hedra inter­link the strands and the chains, building up a three-dimensional framework resembling that of the mineral dumortierite. The OH groups were assigned on basis of bond-valence calculations and crystal chemical considerations.
doi:10.1107/S1600536813010714
PMCID: PMC3647786  PMID: 23723752
13.  Structure cristalline de type alluaudite K0.4Na3.6Co(MoO4)3  
A new triple molybdate K0.4Na3.6Co(MoO4)3 was synthesized using solid-state reaction at 973 K and characterized by X-ray diffraction. The structure is characterized by M 2O10 (M = Co/Na) dimers, which are linked by MoO4 tetra­hedra, forming infinite layers. The latter are connected on one hand by insertion of Mo1O4 tetra­hedra and secondly by sharing corners with Mo2O4 tetra­hedra.
A new triple molybdate, potassium sodium cobalt tris­(molybdate), K0.4Na3.6Co(MoO4)3, was synthesized using solid-state reactions. The Co2+ and one Na+ cation are located at the same general site, each with occupancy 0.5. Another site (site symmetry 2) is occupied by Na+ and K+ cations, with occupancies of 0.597 (7) and 0.402 (6), respectively. The other two Na+ cations and one of the two Mo atoms lie on special positions (site symmetries -1, 2 and 2, respectively). The structure is characterized by M 2O10 (M = Co/Na) dimers, which are linked by MoO4 tetra­hedra, forming infinite layers. The latter are connected firstly by insertion of one type of MoO4 tetra­hedra and secondly by sharing corners with the other type of MoO4 tetra­hedra. This results in an open three-dimensional framework with the cavities occupied by the Na+ and K+ cations. The structure is isotypic with Na3In2As3O12 and Na3In2P3O12. A comparison is made with structures such as K2Co2(MoO4)3 and β-NaFe2(MoO4)3 and their differences are discussed.
doi:10.1107/S2056989014025894
PMCID: PMC4331865
crystal structure; triple molybdate; alluaudite-type
14.  BaMnII 2MnIII(PO4)3  
The title compound, barium trimanganese tris­(ortho­phosphate), was synthesized hydro­thermally. Its structure is isotypic with the lead and strontium analogues AMnII 2MnIII(PO4)3 (A = Pb, Sr). Except for two O atoms on general positions, all atoms are located on special positions. The Ba and one P atom exhibit mm2 symmetry, the MnII atom 2/m symmetry, the MnIII atom and the other P atom .2. symmetry and two O atoms are located on mirror planes. The crystal structure contains two types of chains running parallel to [010]. One chain is linear and is composed of alternating MnIIIO6 octa­hedra and PO4 tetra­hedra sharing vertices; the other chain has a zigzag arrangement and is built up from two edge-sharing MnIIO6 octa­hedra connected to PO4 tetra­hedra by edges and vertices. The two types of chains are linked through PO4 tetra­hedra into an open three-dimensional framework which contains channels parallel to [100] and [010] in which the BaII ions are located. The alkaline earth cation is surrounded by eight O atoms in the form of a slightly distorted bicapped trigonal prism.
doi:10.1107/S1600536813023106
PMCID: PMC3884417  PMID: 24426979
15.  Potassium nickel(II) gallium phosphate hydrate, K[NiGa2(PO4)3(H2O)2] 
The title compound, potassium nickel(II) digallium tris­(phosphate) dihydrate, K[NiGa2(PO4)3(H2O)2], was synthesized hydro­thermally. The structure is constructed from distorted trans-NiO4(H2O)2 octa­hedra linked through vertices and edges to GaO5 trigonal bipyramids and PO4 tetra­hedra, forming a three-dimensional framework of formula [NiGa2(PO4)3(H2O)2]−. The K, Ni and one P atom lie on special positions (Wyckoff position 4e, site symmetry 2). There are two sets of channels within the framework, one running parallel to the [10] direction and the other parallel to [001]. These inter­sect, forming a three-dimensional pore network in which the water mol­ecules coordinated to the Ni atoms and the K+ ions required to charge balance the framework reside. The K+ ions lie in a highly distorted environment surrounded by ten O atoms, six of which are closer than 3.1Å. The coordinated water mol­ecules are within hydrogen-bonding distance to O atoms of bridging Ga—O—P groups.
doi:10.1107/S1600536809015438
PMCID: PMC2977543  PMID: 21583729
16.  Na3Co2(As0.52P0.48)O4(As0.95P0.05)2O7  
The title compound, trisodium dicobalt(II) (arsenate/phosphate) (diarsenate/diphosphate), was prepared by a solid-state reaction. It is isostructural with Na3Co2AsO4As2O7. The framework shows the presence of CoX22O12 (X2 is statistically disordered with As0.95P0.05) units formed by sharing corners between Co1O6 octa­hedra and X22O7 groups. These units form layers perpendicular to [010]. Co2O6 octa­hedra and X1O4 (X1 = As0.54P0.46) tetra­hedra form Co2X1O8 chains parallel to [001]. Cohesion between layers and chains is ensured by the X22O7 groups, giving rise to a three-dimensional framework with broad tunnels, running along the a- and c-axis directions, in which the Na+ ions reside. The two Co2+ cations, the X1 site and three of the seven O atoms lie on special positions, with site symmetries 2 and m for the Co, m for the X1, and 2 and m (× 2) for the O sites. One of two Na atoms is disordered over three special positions [occupancy ratios 0.877 (10):0.110 (13):0.066 (9)] and the other is in a general position with full occupancy. A comparison between structures such as K2CdP2O7, α-NaTiP2O7 and K2MoO2P2O7 is made. The proposed structural model is supported by charge-distribution (CHARDI) analysis and bond-valence-sum (BVS) calculations. The distortion of the coordination polyhedra is analyzed by means of the effective coordination number.
doi:10.1107/S1600536813032029
PMCID: PMC3884975  PMID: 24454150
17.  Cubic ZrW1.75Mo0.25O8 from a Rietveld refinement based on neutron powder diffraction data 
The solid solution in the system Zr–Mo–W–O with composition ZrW1.75Mo0.25O8 (zirconium tungsten molybdenum octa­oxide) was prepared by solid-state reactions as a polycrystalline material. Its structure has cubic symmetry (space group P213) at room temperature. The structure contains a network of corner-sharing ZrO6 octa­hedra (.3. symmetry) and MO4 (M = W, Mo) tetra­hedra (.3. symmetry). Along the main threefold axis of the cubic unit cell, the MO4 tetra­hedra are arranged in pairs forming M 2O8 units in which the M1O4 tetra­hedra have larger distortions in terms of bond distances and angles than the M2O4 tetra­hedra. These units are disordered over two possible orientations, with the M—Oterminal vectors pointing to the [111] or [ ] directions. The reversal of the orientations of the M 2O8 units results from the concerted flips of these units. The time-averaged proportions of flipped and unflipped M 2O8 units were determined and the fraction of unflipped M 2O8 units is about 0.95. The order degree of the M 2O8 unit orientation is about 0.9. During the reversal process, the M-atom site has a migration about 0.93 Å, one of the O-atom sites has a 0.25 Å migration distance, whereas two other O-atom sites migrate marginally (≃ 0.08 Å). The results prove the constraint strategy to be a reasonable approach based on the ratcheting mechanism.
doi:10.1107/S1600536809015281
PMCID: PMC2977542  PMID: 21583728
18.  An ortho­rhom­bic polymorph of the ultraphosphate YP5O14  
Single crystals of yttrium penta­phosphate(V), YP5O14, were obtained by solid-state reaction. The ortho­rhom­bic title compound belongs to the family of ultraphosphates and is the second polymorph of this composition. It is isotypic with its Ho and Er analogues. The structure contains two bridging Q 2-type PO4 tetra­hedra and one branching Q 3-type PO4 tetra­hedron, leading to infinite ultraphosphate ribbons running along the a axis. The coordination polyhedron around the Y3+ cation may be described as distorted bicapped trigonal-prismatic. The YO8 polyhedra are isolated from each other. They are linked by corner-sharing to the O atoms of six Q 2-type and of two Q 3-type PO4 tetra­hedra into a three-dimensional framework.
doi:10.1107/S1600536809007193
PMCID: PMC2968816  PMID: 21582306
19.  Crystal structures of Ca(ClO4)2·4H2O and Ca(ClO4)2·6H2O 
The crystal structures of the tetra- and hexa­hydrate phases of Ca(ClO4)2 consist of Ca2+ ions in distorted square-anti­prismatic environments and of perchlorate tetra­hedra. O—H⋯O hydrogen bonds between water mol­ecules and ClO4 units lead to the formation of a three-dimensional network in the structures.
The title compounds, calcium perchlorate tetra­hydrate and calcium perchlorate hexa­hydrate, were crystallized at low temperatures according to the solid–liquid phase diagram. The structure of the tetra­hydrate consists of one Ca2+ cation eightfold coordinated in a square-anti­prismatic fashion by four water mol­ecules and four O atoms of four perchlorate tetra­hedra, forming chains parallel to [01-1] by sharing corners of the ClO4 tetra­hedra. The structure of the hexa­hydrate contains two different Ca2+ cations, each coordinated by six water mol­ecules and two O atoms of two perchlorate tetra­hedra, forming [Ca(H2O)6(ClO4)]2 dimers by sharing two ClO4 tetra­hedra. The dimers are arranged in sheets parallel (001) and alternate with layers of non-coordinating ClO4 tetra­hedra. O—H⋯O hydrogen bonds between the water mol­ecules as donor and ClO4 tetra­hedra and water mol­ecules as acceptor groups lead to the formation of a three-dimensional network in the two structures. Ca(ClO4)2·6H2O was refined as a two-component inversion twin, with an approximate twin component ratio of 1:1 in each of the two structures.
doi:10.1107/S1600536814024532
PMCID: PMC4257416  PMID: 25552974
crystal structure; low-temperature salt hydrates; perchlorate hydrates; calcium salts; Mars minerals
20.  A new mixed-valence lead(II) mangan­ese(II/III) phosphate(V): PbMnII 2MnIII(PO4)3  
The title compound, lead trimanganese tris(orthophosphate), has been synthesized by hydro­thermal methods. In this structure, only two O atoms are in general positions and all others atoms are in the special positions of the Imma space group. Indeed, the atoms in the Wyckoff positions are namely: Pb1 and P1 on 4e (mm2); Mn1 on 4b (2/m); Mn2 and P2 on 8g (2); O1 on 8h (m); O2 on 8i (m). The crystal structure can be viewed as a three-dimensional network of corner- and edge-sharing PO4 tetra­hedra and MnO6 octa­hedra, building two types of chains running along the b axis. The first is an infinite linear chain, formed by alternating MnIIIO6 octa­hedra and PO4 tetra­hedra which share one vertex. The second chain is built up from two adjacent edge-sharing octa­hedra (MnII 2O10 dimers) whose ends are linked to two PO4 tetra­hedra by a common edge. These chains are linked together by common vertices of polyhedra in such a way as to form porous layers parallel to (001). These sheets are bonded by the first linear chains, leading to the appearance of two types of tunnels, one propagating along the a axis and the other along b. The PbII ions are located within the inter­sections of the tunnels with eight neighbouring O atoms in form of a trigonal prism that is capped by two O atoms on one side. The three-dimensional framework of this structure is compared with similar phosphates such as Ag2Co3(HPO4)(PO4)2 and Ag2Ni3(HPO4)(PO4)2.
doi:10.1107/S1600536813016504
PMCID: PMC3772398  PMID: 24046541
21.  Crystal structures of Na2SeO4·1.5H2O and Na2SeO4·10H2O 
The crystal structures of the 1.5- and 10-hydrates of Na2SeO4 are isotypic with those of the corresponding chromates.
The crystal structures of Na2SeO4·1.5H2O (sodium selenate sesquihydrate) and Na2SeO4·10H2O (sodium selenate deca­hydrate) are isotypic with those of Na2CrO4·1.5H2O and Na2 XSeO4·10H2O (X = S, Cr), respectively. The asymmetric unit of the sesquihydrate contains two Na+ cations, one SeO4 tetra­hedron and one and a half water mol­ecules, the other half being generated by twofold rotation symmetry. The coordination polyhedra of the cations are a distorted monocapped octa­hedron and a square pyramid; these [NaOx] polyhedra are linked through common edges and corners into a three-dimensional framework structure, the voids of which are filled with the Se atoms of the SeO4 tetra­hedra. The structure is consolidated by O—H⋯O hydrogen bonds between coordinating water mol­ecules and framework O atoms. The asymmetric unit of the deca­hydrate consists of two Na+ cations, one SeO4 tetra­hedron and ten water mol­ecules. Both Na+ cations are octa­hedrally surrounded by water mol­ecules and by edge-sharing condensed into zigzag chains extending parallel to [001]. The SeO4 tetra­hedra and two uncoordinating water mol­ecules are situated between the chains and are connected to the chains through an intricate network of medium-strength O—H⋯O hydrogen bonds.
doi:10.1107/S1600536814011799
PMCID: PMC4158548  PMID: 25249853
isotypism; sodium selenate; salt hydrates; crystal structure
22.  The β-modification of trizinc borate phosphate, Zn3(BO3)(PO4) 
Crystals of β-Zn3(BO3)(PO4) have been grown by the Kyropoulos method. The asymmetric unit contains three Zn sites, three B-atom sites (all with symmetry 3), two P sites (both with m symmetry) and nine O-atom sites (four with m symmetry). The fundamental building units of the title structure are isolated BO3 triangles and PO4 tetra­hedra, which are bridged by ZnO4 tetra­hedra or ZnO5 trigonal bipyramids through common O atoms, leading to a three-dimensional framework structure. Some significant structural differences between the β-polymorph and the α-polymorph are discussed.
doi:10.1107/S1600536810051871
PMCID: PMC3050272  PMID: 21522511
23.  NaSr(AsO4)(H2O)9: the (Sr,As) analogue of nabaphite and nastrophite 
The crystal structure of the title compound, sodium strontium orthoarsenate(V) nona­hydrate, is isotypic with NaSr(PO4)(H2O)9 and the minerals nabaphite [NaBa(PO4)(H2O)9] and nastrophite [Na(Sr,Ba)(PO4)(H2O)9]. The Na and Sr atoms are located on threefold rotation axes and are in the centres of slightly distorted Na(H2O)6 octa­hedra and Sr(H2O)9 tricapped trigonal prisms, respectively. A framework structure is established via edge-sharing of these polyhedra. Disordered AsO4 tetra­hedra (with threefold symmetry) are situated in the inter­stitial space of the framework. Although reasonable H-atom positions of the water mol­ecules were not established, close O⋯O contacts between the disordered AsO4 tetra­hedra and the water mol­ecules suggest strong O—H⋯O hydrogen bonding.
doi:10.1107/S1600536809040355
PMCID: PMC2971340  PMID: 21578034
24.  An open-framework borophosphate, LiCu2BP2O8(OH)2  
The open-framework alkaline-earth metal borophosphate, lithium dicopper(II) borophosphate dihydroxide, LiCu2BP2O8(OH)2, was synthesized hydro­thermally. Its structure may be regarded as a layer formed via BO4 and PO4 tetra­hedra bonding together with distorted CuO6 and LiO6 octa­hedral units. Each P atom is connected to B, Li and Cu atoms through a bridging O atom. The B atom lies on a crystallographic twofold axis and the Li atom lies on a center of symmetry. The two metal centers are connected to each other by Cu—O—Li bonds.
doi:10.1107/S1600536809015554
PMCID: PMC2977544  PMID: 21583730
25.  Lithio­marsturite, LiCa2Mn2Si5O14(OH) 
Lithio­marsturite, ideally LiCa2Mn2Si5O14(OH), is a member of the pectolite–pyroxene series of pyroxenoids (hydro­pyroxenoids) and belongs to the rhodonite group. A previous structure determination of this mineral based on triclinic symmetry in space group P by Peacor et al. [Am. Mineral. (1990), 75, 409–414] converged with R = 0.18 without reporting any information on atomic coordinates and displacement param­eters. The current study redetermines its structure from a natural specimen from the type locality (Foote mine, North Carolina) based on single-crystal X-ray diffraction data. The crystal structure of lithio­marsturite is characterized by ribbons of edge-sharing CaO6 and two types of MnO6 octa­hedra as well as chains of corner-sharing SiO4 tetra­hedra, both extending along [110]. The octa­hedral ribbons are inter­connected by the rather irregular CaO8 and LiO6 polyhedra through sharing corners and edges, forming layers parallel to (1), which are linked together by the silicate chains. Whereas the coordination environments of the Mn and Li cations can be compared to those of the corresponding cations in nambulite, the bonding situations of the Ca cations are more similar to those in babingtonite. In contrast to the hydrogen-bonding scheme in babingtonite, which has one O atom as the hydrogen-bond donor and a second O atom as the hydrogen-bond acceptor, our study shows that the situation is reversed in lithio­marsturite for the same two O atoms, as a consequence of the differences in the bonding environments around O atoms in the two minerals.
doi:10.1107/S1600536811047581
PMCID: PMC3238580  PMID: 22199471

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