Group-subgroup Relations Research Articles

Symmetry relationships between hybrid lead halide perovskite-derived phases: vacancy ordering as a key classification factor

For the first time, based on symmetry group–subgroup relation-ships, an approach is presented to create a clear classification of hybrid lead halides with low-dimensional vacancy-ordered perovskite-related crystal structures to facilitate the identification and prediction of hybrid lead-halide materials that have desired properties.

Ordering by cation replacement in the system Na2-xLixGa7.

Samples of the pseudo-binary system Na2-xLixGa7 (x ≤ 1) were synthesized from the elements at 300 °C in sealed Ta ampoules or by the reaction of Na2Ga7 with LiCl. The peritectic formation temperature decreases with increasing Li content from 501(2) °C (x = 0) to 489(2) °C (x = 1). The boundary compositions Na2Ga7 and Na1Li1Ga7 crystallize with different structure types related by a group-subgroup relation. While the Na-rich compositions (x ≤ 0.5) represent a substitutional solid solution (space group Pnma), the Li-rich compositions feature an unconventional replacement mechanism in which Li atoms occupying interstitial positions induce vancancies at the Na positions (space group Cmce). The crystal structure of Na1Li1Ga7 (a = 8.562(1) Å, b = 14.822(2) Å, c = 11.454(2) Å; Z = 8) was determined from X-ray single-crystal diffraction data, and reveals an anionic framework comprising 12-bonded Ga12 icosahedra and 4-bonded Ga atoms, with alkali-metal atoms occupying channels and cavities. The arrangement of cations makes NaLiGa7 a new structure type within the MgB12Si2 structure family. Band structure calculations for the composition NaLiGa7 predict semiconducting behavior consistent with the balance [Na+]2[Li+]2[(Ga12)2-][Ga-]2, considering closo Wade clusters [(12b)Ga12]2- and Zintl anions [(4b)Ga]-. Susceptibility measurements indicate temperature-independent diamagnetic behavior.

Larger recovery strains in a CoNiSi alloy due to enhanced reversibility of FCC⇌HCP martensitic transformation

It is not clear how coherent ordered nanoparticles affect characteristics of FCC⇌HCP martensitic transformation in disordered alloys. In this paper, the precipitations of coherent ordered nanoparticles and their effects on the FCC⇌HCP martensitic transformation were systematically investigated in a solution-treated Co24Ni6Si shape memory alloy. The results showed that both the coherent ordered L12 structured and L11 structured nanoparticles could precipitate even in the solution-treated Co24Ni6Si alloy followed by water quenching. The L12 structured nanoparticles strongly hindered the occurrence of stress-induced HCP martensitic transformation and its reverse transformation, but their hindrance dramatically reduced with the decrease of size when below 1.5 nm. The coherent ordered nanoparticles did not convert the non-thermoelastic FCC⇌HCP martensitic transformation to the thermoelastic ones owing to the absence of group-subgroup relation between the parent phase and martensite. A maximal recovery strain of 6.7 % was obtained after only aging at 1073 K for 1 min. This value was 1∼2 % higher than the reported maximal ones in conventional FeMnSi-based alloys subjected to complicated thermomechanical treatments. The enhanced reversibility of HCP martensitic transformation could account for this larger recovery strain by the ultrafine coherent ordered L12 structured nanoparticles.

Dimorphism of [Bi2O2(OH)](NO3) - the ordered Pna21 structure at 100 K.

The re-investigation of [Bi2O2(OH)](NO3), dioxidodibismuth(III) hydroxide nitrate, on the basis of single-crystal X-ray diffraction data revealed an apparent structural phase transition of a crystal structure determined previously (space group Cmc21 at 173 K) to a crystal structure with lower symmetry (space group Pna21 at 100 K). The Cmc21 → Pna21 group-subgroup relationship between the two crystal structures is klassengleiche with index 2. In contrast to the crystal structure in Cmc21 with orientational disorder of the nitrate anion, disorder does not occur in the Pna21 structure. Apart from the disorder of the nitrate anion, the general structural set-up in the two crystal structures is very similar: [Bi2O2]2+ layers extend parallel to (001) and alternate with layers of (OH)- anions above and (NO3)- anions below the cationic layer. Whereas the (OH)- anion shows strong bonds to the BiIII cations, the (NO3)- anion weakly binds to the BiIII cations of the cationic layer. A rather weak O-H⋯O hydrogen-bonding inter-action between the (OH)- anion and the (NO3)- anion links adjacent sheets along [001].

The International Tables Symmetry Database

The International Tables Symmetry Database (https://symmdb.iucr.org/), which is part of International Tables for Crystallography, is a collection of individual databases of crystallographic space-group and point-group information with associated programs. The programs let the user access and in some cases interactively visualize the data, and some also allow new data to be calculated `on the fly'. Together these databases and programs expand upon and complement the symmetry information provided in International Tables for Crystallography Volume A, Space-Group Symmetry, and Volume A1, Symmetry Relations between Space Groups. The Symmetry Database allows users to learn about and explore the space and point groups, and facilitates the study of group–subgroup relations between space groups, with applications in determining crystal-structure relationships, in studying phase transitions and in domain-structure analysis. The use of the International Tables Symmetry Database in all these areas is demonstrated using several examples.

Investigations on the polymorphism of K4CaSi6O15 at elevated temperatures.

In the present study, single crystals and polycrystalline material of K4CaSi6O15 were prepared from solid-state reactions between stoichiometric mixtures of the corresponding oxides/carbonates. Heat capacity (C p) measurements above room temperature using a differential scanning calorimeter indicated that two thermal effects occurred at approximately T 1=462 K and T 2=667 K, indicating the presence of structural phase transitions. The standard third-law entropy of K4CaSi6O15 was determined from low-temperature C p's measured by relaxation calorimetry using a Physical Properties Measurement System and amounts to S°(298K)=524.3±3.7 J·mol-1·K-1. For the 1st transition, the enthalpy change ΔH tr1=1.48kJ·mol and the entropy change ΔS tr1=3.25 J·mol-1·K-1, whereas ΔH tr2=3.33kJ·mol-1 and ΔS tr2=5.23 J·mol-1·K-1 were determined for the 2nd transition. The compound was further characterized by in-situ single-crystal X-ray diffraction between ambient temperature and 1063 K. At 773 K, the high-temperature phase stable above T 2 has the following basic crystallographic data: monoclinic symmetry, space group P21/c, a=6.9469(4) Å, b=9.2340(5) Å, c=12.2954(6) Å, β=93.639(3)°, V=787.13(7) Å3, Z=2. It belongs to the group of interrupted framework silicates and is based on tertiary (Q3-type) [SiO4]-tetrahedra. Together with the octahedrally coordinated Ca-cations, a three-dimensional mixed polyhedral network structure is formed, in which the remaining K-ions provide charge balance by occupying voids within the net. The intermediate temperature modification stable between T 1 and T 2 shows a (3+2)-dimensional incommensurately modulated structure that is characterized by the following q-vectors: q1=(0.057, 0.172, 0.379), q2=(-0.057, 0.172, -0.379). The crystal structures of the high- and the previously studied ambient temperature polymorph (space group Pc) are topologically equivalent and show a group-subgroup relationship. The index of the low- in the high-symmetry group is six and involves both, losses in translation as well as point group symmetry. The distortion is based on shifts of the different atom species and tilts of the 4- and 6-fold coordination polyhedra. Actually, for some of the oxygen atoms, the displacements exceed 0.5 Å. A more detailed analysis of the distortions relating to both structures has been performed using mode analysis, which revealed that the primary distortion mode transforms according to the Λ1 irreducible representation of P21/c. However, other modes with smaller distortion amplitudes are also involved.

Twinned single crystal structure of Li4P2S6

Abstract Yellow needles-like single crystals of Li4P2S6 were obtained serendipitously during the preparation of Li7P3S10O. The twinned crystal structure of Li4P2S6 was determined from single-crystal X-ray diffraction data [wR(F 2) = 0.069, 716 reflections, 40 variables]. Li4P2S6 crystallizes in the trigonal system, space group P 3 ‾ m 1 $P\overline{3}m1$ (N° 164), a = 10.5042(8) Å, c = 6.5837(6) Å, V = 629.11(9) Å3 and Z = 2. The lithium octahedra form a [Li4S6]8− honeycomb-like structure within which diphosphate units are located. The comparison of our crystal structure to those of P63/mcm-, P 3 ‾ 1 m $P\overline{3}1m$ -, and P321-Li4P2S6 demonstrated group-subgroup relationships and associated the disorder or order of the phosphorus atoms within the identical [Li4S6]8− 3d-frameworks to the choice of the unit cell (the subcell with a ∼ 6.07 Å vs. the supercell with a ∼ 10.5 Å).

Extensive Polymorphism in the Molecular Ferroelectric 18-Crown-6 Oxonium Tetrachloro-Gallium(III).

The materials property of ferroelectricity is intimately linked with symmetry-changing phase transitions. Characterizing such transitions is therefore essential for understanding molecular ferroelectrics. In this paper, we explore the temperature and thermal history dependence of polymorphic phase transitions in the multiaxial molecular ferroelectric 18-crown-6 oxonium tetrachloro-gallium(III). We have solved the structures of two previously suggested polymorphs (D and Y) ab initio from high-temperature powder diffraction data. We also report the structure of a new polymorph (X) using low-temperature powder diffraction data and identify a fifth (W) that can form on cooling. These polymorphs can be related using two distinct group-subgroup trees. Structure types A-C observed in this and related compounds can be derived from high-temperature polymorph D by group-subgroup relationships. The X and Y polymorphs can be described as child structures of a hypothetical polymorph Z using a molecular rotational distortion mode description. The ferroelectric properties of the various polymorphs can be rationalized based on our structural findings.

Low-temperature crystal structures of the solvent dimethyl carbonate

Dimethyl carbonate (DMC) is an important industrial solvent but is additionally a common component of liquid lithium-ion battery electrolytes. Pure DMC has a melting point of 277 K, so encountering solidification under outdoor climatic conditions is very likely in many locations around the globe. Even eutectic, ethylene carbonate:dimethyl carbonate commercial LiPF6 salt electrolyte formulations can start to solidify at temperatures around 260 K with obvious consequences for their performance. No structures for crystalline DMC are currently available which could be a hindrance for in situ battery studies at reduced temperatures. A time-of-flight neutron powder diffraction study of the phase behavior and crystal structures of deuterated DMC was undertaken to help fill this knowledge gap. Three different orthorhombic crystalline phases were found with a previously unreported low-temperature phase transition around 50–55 K. The progression of Pbca → Pbcm → Ibam space groups follow a sequence of group–subgroup relationships with the final Ibam structure being disordered around the central carbon atom.

Theoretical and 27Al NMR Spectroscopic Investigations of Binary Intermetallic Alkaline-Earth Aluminides.

The binary alkaline-earth aluminides AEAl2 (AE = Ca and Sr) and AEAl4 (AE = Ca-Ba) have been synthesized from the elements and investigated via powder X-ray diffraction experiments. CaAl2 adopts the cubic MgCu2-type structure (Fd3̅m), while SrAl2 crystallizes in the orthorhombic KHg2-type (Imma). LT-CaAl4 crystallizes with the monoclinic CaGa4-type (C2/m), while HT-CaAl4, SrAl4, and BaAl4 adopt the tetragonal BaAl4-type structure (I4/mmm). The close structural relation of the two CaAl4 polymorphs was established using a group-subgroup relation in the Bärnighausen formalism. In addition to the room-temperature and normal pressure phase of SrAl2, a high-pressure/high-temperature phase has been prepared using multianvil techniques, and its structural and spectroscopic parameters were determined. Elemental analysis by inductively coupled plasma mass spectrometry showed that no significant impurities with other elements besides the weighed ones are present and the chemical compositions match the synthesized ones. The title compounds have been furthermore investigated by 27Al solid-state magic angle spinning NMR experiments to validate the crystal structure and to gain information about the influence of the composition on the electron transfer and the NMR characteristics. This has also been investigated from a quantum chemical point of view using Bader charges, while the stabilities of the binary compounds in the three phase diagrams (Ca-Al, Sr-Al and Ba-Al) have been studied by calculations of formation energies per atom.

MAl4Ir2 (M = Ca, Sr, Eu): superstructures of the KAu4In2 type

Three new iridium aluminum intermetallics CaAl4Ir2, SrAl4Ir2, and EuAl4Ir2 were synthesized from the elements using silica or tantalum ampoules. They crystallize in the tetragonal crystal system with space group P4/ncc and lattice parameters of a = 782.20(1) and c = 779.14(2) pm for CaAl4Ir2, a = 797.62(1) and c = 772.75(2) pm for SrAl4Ir2, and finally a = 791.78(5) and c = 773.31(5) pm for EuAl4Ir2. All compounds crystallize isostructurally and adopt a new structure type that can be derived from the KAu4In2 type structure. To compare the structures from a crystallographic point of view, a group–subgroup relation between KAu4In2 and EuAl4Ir2 as well as KAu4In2 and KAu4Sn2 have been established using the Bärnighausen formalism. Finally, quantum-chemical calculations have been conducted, showing that in all three title compounds, a polyanionic [Al4Ir2]δ– network exists with significant (polar) bonding interactions, while the respective Caδ+, Srδ+, and Euδ+ cations are located in octagonal channels.Graphical abstract

Merohedral Mechanism Twining Growth of Natural Cation-Ordered Tetragonal Grossular

Garnet supergroup minerals are in the interest of different applications in geology, mineralogy, and petrology and as optical material for material science. The growth twins of natural tetragonal grossular from the Wiluy River, Yakutia, Russia, were investigated using single-crystal X-ray diffraction, optical studies, Raman spectroscopy, microprobe, and scanning electron microscopy. The studied grossular is pseudo-cubic (a = 11.9390 (4), c = 11.9469 (6) Å) and birefringent (0.01). Its structure was refined in the Ia3¯d, I41/acd, I41/a, and I4¯2d space groups. The I41/a space group was chosen as the most possible one due to the absence of violating reflections and ordering of Mg2+ and Fe3+ in two independent octahedral sites, which cause the symmetry breaking according to the group–subgroup relation Ia3¯d → I41/a. Octahedral crystals of (H4O4)4−-substituted grossular are merohedrally twinned by twofold axis along [110]. The mechanism of twining growth led to the generation of stacking faults on the (110) plane and results in the formation of crystals with a long prismatic habit.

Evaluation of phase relationship in W-Fe-C ternary system through symmetry principles and first-principles calculation

Solid state phase reactions are complicated and sometimes a black box because of the copious reactants and versatile pathways. In this work, the phase relationship in W-Fe-C ternary system is systematically studied by means of the symmetry principles in crystal chemistry and first-principles thermodynamics. According to group-subgroup relation, ternary carbides FexWxC (x = 6, 3, and 2) are likely to form via the intermediate W2C and FeW3C phases at C-rich condition. At C-poor condition, those ternary carbides are prone to be transformed from Fe7W6 with residual C element in the matrix or C from Fe3C in steel. Based on percolation theory, we find the direct phase transition from Fe6W6C to Fe3W3C undergoes with transient Fe2W2C. The calculated Gibbs reaction free energy is in good agreement with various experimental observation and clarifies the discrepancies and controversaries among different experiments in the literature. The success in predicting the phase relationship of W-Fe-C ternary system suggests that the symmetry principles in crystal chemistry combined with first-principles calculations not only unravel the intrinsic mechanism behind the phase reaction, but also should be incorporated into future designs of covalent materials, such as carbides, nitrides, and oxides.

Symmetry and strain analysis of combined electronic and structural instabilities in tungsten trioxide, WO3

Lattice parameter data from the literature have been used to provide a complete description of spontaneous strain variations across each of the six known phase transitions of WO3 in the temperature interval 5–1273 K. Analysis of strain/order parameter coupling reveals the character of each phase transition, a unified description of strain across the full temperature range, the relationship between strain and electronic effects, and new insights into the strain gradients likely to be present in each of the different domain walls that develop in four different ferroelastic phases. Tetragonal and orthorhombic shear strains have values of 4%–6% and 2%–3%, respectively, and are dominated by coupling with the order parameter for antiferroelectric-type displacements. Conversely, shear strains, e4, e5, and e6, of up to 2% are controlled by octahedral tilting. Changes in electronic structure and properties have been related back to the susceptibility of W6+ to develop cooperative second-order-Jahn–Teller distortions. Proximity to tilt instabilities along with group–subgroup relationships in the P4/nmm parent structure results in two overlapping sequences of structural phase transitions, which differ in the form of their electronic structure. The possibility of a ground state structure in space group P21/c can be rationalized in terms of the efficiency by which different combinations of shearing and tilting of the WO6 octahedra can reduce the unit cell volume and would imply that WO3 has a re-entrant phase transition. Gradients in up to three order parameters coupled with gradients in strain of up to 12% across ferroelastic domain walls indicate that the different ferroelastic phases of WO3 should have domain walls with varied and potentially exotic electronic properties for device applications such as in nanoelectronics and neuromorphic computing.

Study of the Temperature- and Pressure-Dependent Structural Properties of Alkali Hydrido-closo-borate Compounds.

In this work, we report on the structural properties of alkali hydrido-closo-(car)borates, a promising class of solid-state electrolyte materials, using high-pressure and temperature-dependent X-ray diffraction experiments combined with density functional theory (DFT) calculations. The mechanical properties are determined via pressure-dependent diffraction studies and DFT calculations; the shear moduli appear to be very low for all studied compounds, revealing their high malleability (that can be beneficial for the manufacturing and stable cycling of all-solid-state batteries). The thermodiffraction experiments also reveal a high coefficient of thermal expansion for these materials. We discover a pressure-induced phase transition for K2B12H12 from Fm3̅ to Pnnm symmetry around 2 GPa. A temperature-induced phase transition for Li2B10H10 was also observed for the first time by thermodiffraction, and the crystal structure determined by combining experimental data and DFT calculations. Interestingly, all phases of the studied compounds (including newly discovered high-pressure and high-temperature phases) may be related via a group–subgroup relationship, with the notable exception of the room-temperature phase of Li2B10H10.

Ion-Exchange-Induced Transformation and Mechanism of Cooperative Crystal Chemical Adaptation in Sitinakite: Theoretical and Experimental Study

The microporous titanosilicate sitinakite, KNa2Ti4(SiO4)2O5(OH)·4H2O, was first discovered in the Khibiny alkaline massif. This material is also known as IONSIV IE-911 and is considered as one of the most effective sorbents for Cs+ and Sr2+ from water solutions. We investigate a mechanism of cooperative crystal chemical adaptation caused by the incorporation of La3+ ions into sitinakite structure by the combination of theoretical (geometrical–topological analysis, Voronoi migration map calculation, structural complexity calculation) and empirical methods (PXRD, SCXRD, Raman spectroscopy, scanning electron microscopy). The natural crystals of sitinakite (a = 7.8159(2), c = 12.0167(3) Å) were kept in a 1M solution of La(NO3)3 for 24 h. The ordering of La3+ cations in the channels of the ion-exchanged form La3+Ti4(SiO4)2O5(OH)·4H2O (a = 11.0339(10), b = 11.0598(8), c = 11.8430(7) Å), results in the symmetry breaking according to the group–subgroup relation P42/mcm → Cmmm.

Structure relations in the family of the solid solution Hf x Zr1−x O2

Abstract Hafnium Zirconium Oxide Hf x Zr1−x O2 is a potentially ferroelectric material with great perspectives in semiconductor applications, due to its compatibility with silicon technologies and its low toxicity. Despite its chemical simplicity, the solid solution Hf x Zr1−x O2 comprises a large variety of different phases. We compiled a complete list of experimentally and theoretically reported Hf x Zr1−x O2 structures. All of them are symmetrically related to the common aristotype with Fluorite type structure. The symmetry relationships between those structures have been determined and are presented in a Bärnighausen-like tree. Interestingly, not all symmetry reductions follow the conventional group-subgroup relations and involve severe atomic shifts. Further, the structures were compared to each other in detail regarding the dimensionality of atomic shifts and the accompanied lattice distortions. Finally, the information provided by the Bärnighausen-like tree was used to transform the indices of a reflection before and after a phase transition. This conversion allows the study of (dis)appearing reflections during phase transitions.

Dimorphism of MnHAsO4(H2O): natural monoclinic krautite and its synthetic triclinic modification

Abstract The crystal structure of natural krautite, MnHAsO4(H2O), was re-evaluated from a cotype specimen, confirming the previously reported monoclinic symmetry for this mineral (space group P21, Z = 8, a = 8.0093(5), b = 15.9372(10), c = 6.8065(4) Å, β = 96.534(2)° at room temperature, 5662 structure factors, 302 parameters, R1 = 0.0295, wR2 = 0.0770). Although hydrogen atoms could not be located from the single crystal X-ray diffraction study, the higher accuracy and precision of the results allowed to derive the hydrogen-bonding scheme (O⋯O = 2.55–2.90 Å) in the crystal structure of krautite. Crystals of synthetic MnHAsO4(H2O) were grown by mixing aqueous solutions of NH4H2AsO4 and MnSO4 and keeping the formed gel at 105 °C for several days. The obtained triclinic crystals were systematically and polysynthetically twinned by contact on (010). Separation of reflections from two individual domains made it possible to determine and refine the crystal structure (space group P 1 ‾ $P\overline{1}$ , Z = 8, a = 8.0105(16), b = 15.991(4), c = 6.8029(12) Å, α = 92.635(8), β = 96.534(2), γ = 90.151(8)° at room temperature, 7226 structure factors, 255 parameters, R1 = 0.0445, wR2 = 0.1381). The triclinic polymorph of MnHAsO4(H2O) does not show a direct group-subgroup relation with monoclinic krautite. Triclinic MnHAsO4(H2O) is closely related with other triclinic M IIHAsO4(H2O) (M = Co, Cu, Zn, Mg) mineral phases. Quantitative structural comparisons between the five M IIHAsO4(H2O) compounds revealed a high similarity between the Mn and Co members, and between the Zn and Mg members, respectively. Subtle distinctions between the two pairs are ascribed to a different hydrogen-bonding scheme. Although the Cu member has a similar hydrogen-bonding scheme as the Mn and Co pair, its structural similarity with triclinic MnHAsO4(H2O) is low due to the strain of the crystal structure caused by the Jahn-Teller distortions of the [CuO6] octahedra.

The layer silicate Cs2SnIVSi6O15.

Single crystals of Cs2SnSi6O15, dicaesium tin(IV) hexa-silicate, were serendipitously obtained from a CsCl/NaCl flux at 923 K, starting from mixtures of CaO, SnO and TeO2 in a closed silica ampoule. The crystal structure of Cs2SnSi6O15 is constructed from {Si6O15}6- layers extending parallel to (101), and CsI cations with a coordination number of eleven as well as isolated [SnO6] octa-hedra situated between the silicate layers. Each of the nine different SiO4 tetra-hedra in the silicate layer has a connectedness of Q 3 (three bridging and one terminal O atom), which leads to the formation of five- and eight-membered rings. The same type of silicate layer is found in the crystal structure of the mineral zeravshanite. Comparison with other silicates of the type Cs2 M IVSi6O15 (M IV = Ti, Zr, Th, U) revealed a klassengleiche group-subgroup relationship of index 2 between Cs2ZrSi6O15 (Z = 6, space group C2/m) and Cs2SnSi6O15 (Z = 12, space group I2/c).

On the incorporation of strontium into the crystal structure of bredigite: structural effects and phase transition

Sr-substitution in the crystal structure of bredigite has been studied in detail. Samples of a hypothetical solid-solution series with nominal composition Ca7-xSrxMg[SiO4]4 (x = 0, 2, …,7) have been prepared from sinter reactions in the temperature range between 1275 and 1325 °C and characterized using powder and single-crystal X-ray diffraction. Synthesis runs between x = 1 and x = 4 resulted in compounds with increasing Sr contents, for which single-crystal diffraction studies revealed the following Sr/(Sr + Ca) atomic ratios: 0.133,0.268, 0.409 and 0.559. They are isostructural to the pure calcium end-member (x = 0) and adopt the orthorhombic space group Pnnm. Evolution of the unit-cell parameters and cell volumes of the solid-solution series are defined by linear or nearly linear trends when plotted against the Sr/(Sr + Ca) atomic ratio. Replacement of calcium with strontium atoms on the different sites shows clear preferences for specific positions. For the experiment with x = 5, formation of bredigite-related single-crystals with Sr/(Sr + Ca) = 0.675 was observed. These samples, however, exhibited a halved c lattice parameter when compared with the corresponding value in the Pnnm structure, pointing to a compositionally induced phase transition somewhere in region between Sr/(Sr + Ca) = 0.559 and 0.675. The crystal structure of this new phase with composition Ca2.32Sr4.82Mg0.86[SiO4]4 was successfully determined in space group Pbam. Basic crystallographic data are as follows: a = 18.869(2) Å, b = 6.9445(8) Å, c = 5.5426(6) Å, V = 726.28(14) Å3, Z = 2. Structure determination was accomplished using charge flipping. Subsequent least-squares refinements resulted in a residual of R(|F|) = 2.70% for 822 independent reflections and 87 parameters. The Pbam- and the Pnnm-structures are in a group-subgroup relationship and topologically related. Both are based on so-called pinwheel-like MSi6O24 clusters consisting of a central magnesium-dominated [MO6]-octahedron as well as six attached [SiO4]-tetrahedra. The clusters are linked into chain-like elements running along [001]. Linkage between the chains is provided by mixed Sr/Ca positions with 6 to 10 oxygen ligands. Differences between the two phases result from changes in Sr-Ca site occupancies in combination with displacements of the atoms and tilts of the tetrahedra. The distortion pattern has been studied using group-theoretical methods including mode analysis. Notably, for the samples with x = 6 and x = 7—the latter corresponding to the hypothetical pure strontium end-member composition—no bredigite-type phases could be identified, indicating that there is an upper limit for the Sr-uptake.