Phase transition and structures of the twinned low‐temperature phases of (Et4N)[ReS4]

The title compound, tetraethylammonium tetrathiorhenate, [(C₂H₅)₄N][ReS₄], has, at room temperature, a disordered structure in the space group P6₃mc (Z = 2, α‐phase). A phase transition to the monoclinic space group P2₁ (Z = 2, γ‐phase) at 285 K leads to a pseudo‐merohedral twin. The high deviation from the hexagonal metric causes split reflections. However, the different orientations could not be separated, but were integrated using a large integration box. Rapid cooling to 110–170 K produces a metastable β‐phase (P6₃, Z = 18) in addition to the γ‐phase. All crystals of the β‐phase are contaminated with the γ‐phase. Additionally, the crystals of the β‐phase are merohedrally twinned. In contrast to the α‐phase, the β‐ and γ‐phases do not show disorder.     

The structural phase transition in SrV6O11

Single‐crystal X‐ray diffraction and specific heat studies establish that strontium hexavanadium undecaoxide, SrV₆O₁₁, undergoes a P6₃/mmc to inversion twinned P6₃mc structural transition as the temperature is lowered through 322 K. The P6₃/mmc and P6₃mc structures have been determined at 353 K and at room temperature, respectively. For the room‐temperature structure, seven of the ten unique atoms lie on special positions, and for the 353 K structure all of the seven unique atoms sit on special positions. The P6₃/mmc to P6₃mc structural phase transition, accompanied by a magnetic transition, is a common characteristic of AV₆O₁₁ compounds, independent of the identity of the A cations.     

Hexagonal high‐temperature form of aluminium phosphate tridymite from X‐ray powder data

Similar to silica tridymite, AlPO₄ tridymite shows a sequence of displacive phase transitions resulting in a dynamically disordered hexagonal high‐temperature modification. Rietveld refinement reveals that the thermal motions of the tetrahedra can be described either by strongly anisotropic displacement parameters for oxy­gen or by split O atoms. Due to the ordered distribution of aluminium and phospho­rus over alternating tetrahedra, the space group symmetry of high‐temperature AlPO₄ tridymite is reduced with respect to SiO₂ tridymite from P6₃/mmc to P6₃mc.     

Synthesis,Crystal Structure Determination and Fe/Rh Site Preference in the New Ternary Boride FeRh6B3

Single crystals of FeRh₆B₃ were synthesized by arc‐melting the elements in a water‐cooled copper crucible under an argon atmosphere. The new silver‐like compound, structurally characterized by single‐crystal X‐ray analysis, crystallizes in the hexagonal Th₇Fe₃ structure type (space group P6₃mc, no. 186, Z = 2; a = 7.4135(6) Å, c = 4.7168(6) Å, R₁ = 0.040, wR₂ = 0.083 for all 376 reflections and 21 parameters). The structure consists of layers of boron‐centered trigonal prisms of rhodium/iron atoms and one‐dimensional strings of face‐sharing octahedral Rh₆ clusters.     

The low‐temperature phase of di‐tert‐butylsilanediol

The crystal structure of di‐tert‐butyl­silanediol, C₈H₂₀O₂Si, has a reversible phase transition at 211 (2) K. The orthorhombic high‐temperature structure has space group Ibam, with Z′ = , and shows a disordered hydrogen‐bonding system. The low‐temperature structure, determined at 143 (2) K, has a twinned monoclinic cell, with space group C2/c and Z′ = 2, and shows an ordered hydrogen‐bonding system.     

Concealed Cyclotrimeric Polymorph of Lithium 2,2,6,6‐Tetramethylpiperidide Unconcealed: X‐Ray Crystallographic and NMR Spectroscopic Studies

Lithium 2,2,6,6‐tetramethylpiperidide (LiTMP), one of the most important polar organometallic reagents both in its own right and as a key component of ate compositions, has long been known for its classic cyclotetrameric (LiTMP)₄ solid‐state structure. Made by a new approach through transmetalation of Zn(TMP)₂ with tBuLi in n‐hexane solution, a crystalline polymorph of LiTMP has been uncovered. X‐ray crystallographic studies at 123(2) K revealed this polymorph crystallises in the hexagonal space group P6₃/m and exhibited a discrete cyclotrimeric (C_3h) structure with a strictly planar (LiN)₃ ring containing three symmetrically equivalent TMP chair‐shaped ligands. The molecular structure of (LiTMP)₄ was redetermined at 123(2) K, because its original crystallographic characterisation was done at ambient temperature. This improved redetermination confirmed a monoclinic C2/c space group with the planar (LiN)₄ ring possessing pseudo (non‐crystallographic) C_4h symmetry. Investigation of both metalation and transmetalation routes to LiTMP under different conditions established that polymorph formation did not depend on the route employed but rather the temperature of crystallisation. Low‐temperature (freezer at ?35 °C) cooling of the reaction solution favoured (LiTMP)₃; whereas high‐temperature (bench) storage favoured (LiTMP)₄. Routine ¹H and ¹³C NMR spectroscopic studies in a variety of solvents showed that (LiTMP)₃ and (LiTMP)₄ exist in equilibrium, whereas ¹H DOSY NMR studies gave diffusion coefficient results consistent with their relative sizes.     

Synthesis,Characterization, and Magnetic Properties of the New Boride Solid Solutions M0.5Ru6.5B3 (M = Cr,Mn, Co,Ni)

Powder samples and single crystals of the borides M_0.5Ru_6.5B₃ (M = Cr, Mn, Co, Ni) were synthesized by arc‐melting the elements in a water‐cooled copper crucible under argon. The new phases were structurally characterized by single‐crystal and powder X‐ray diffraction as well as EDX‐Analyses. They crystallize in the hexagonal Th₇Fe₃ structure type (space group P6₃mc, no. 186, Z = 2) and a pronounced site preferential M/Ru substitution is observed. Magnetic properties of the compounds were investigated and Pauli paramagnetism was observed in all cases. However, a strong temperature dependency is subsequently observed in Mn_0.5Ru_6.5B₃ below 250 K, but no hint of magnetic ordering was found.     

Structure of Plastic Crystalline Succinonitrile: High‐Resolution in situ Powder Diffraction

The temperature dependent (150–290 K) crystal structure of the low‐temperature α‐phase, and high temperature β‐phase, of succinonitrile has been determined by high resolution in situ powder diffraction. The α‐phase has a monoclinic unit cell that contains four gauche molecules and belongs to the P2₁/a space group. The crystal undergoes a reversible first‐order phase transition at 233 K into the high temperature β‐phase. The lattice parameters increase with temperature and the phase transition leads to an abrupt 6.7 % increase in volume. The β‐phase crystallizes into a bcc‐structure that belongs to the space group. The high temperature phase; however, is a highly disordered plastic crystal at room temperature that contains both gauche and trans molecules. The non‐linearity in the overall isotropic temperature‐factor indicates other possible phase transitions in the temperature range of 233–250 K.     

Synchrotron‐radiation study of the two‐leg spin‐ladder (VO)2P2O7 at 120 K

The crystal structure of the ambient‐pressure phase of vanadyl pyrophosphate, (VO)₂P₂O₇, has been precisely determined at 120 K from synchrotron X‐ray diffraction data measured on a high‐quality single crystal. The structure refinement unambiguously establishes the orthorhombic space group Pca2₁ as the true crystallographic symmetry. Moreover, it improves the accuracy of previously published atomic coordinates by one order of magnitude, and provides reliable anisotropic displacement parameters for all atoms. Along the a axis, the structure consists of infinite two‐leg ladders of vanadyl cations, (VO)²⁺, which are separated by pyrophosphate anions, (P₂O₇)^4?. Parallel to the c axis, the unit cell comprises two alternating crystallographically inequivalent chains of edge‐sharing VO₅ square pyramids bridged by PO₄ double tetrahedra. No structural phase transition has been observed in the temperature range between 300 and 120 K.     

Zn‐rich Zincides of the Ternary System Ca–Mg–Zn: Synthesis,Crystal structure,Chemical Bonding

In the course of a study on the role of magnesium in polar zincides of the heavier alkaline‐earth elements, three intermetallic phases of the ternary system Ca–Mg–Zn were synthesized from melts of the elements and their structures were determined by means of single‐crystal X‐ray data. Starting from the binary zincide CaZn₁₁, the phase width of the BaCd₁₁‐type structure reaches up to the fully ordered stoichiometric compound CaMgZn₁₀ [tI48, space group I4₁/amd, a = 1082.66(6), c = 688.95(5) pm, Z = 4, R₁ = 0.0239]. The new compound CaMgZn₅ (oP28, space group Pnma, a = 867.48(3), b = 530.37(5), c = 1104.45(9) pm, Z = 4, R₁ = 0.0385) crystallizes in the CeCu₆‐type structure, exhibits no Mg/Zn phase width and has no binary border equivalent in the system Ca–Mg–Zn. Similar to the situation in CaMgZn₁₀, one M position of the aristotype has a slightly larger coordination sphere (CN = 14) and is accordingly occupied by the larger Mg atoms. The third phase, Ca_2+xMg_6–x–yZn_15+y (hP92, space group P6₃/mmc, a = 1476.00(5), c = 881.01(4) pm, Z = 4, R₁ = 0.0399 for Ca_2.67Mg_5.18Zn_15.15) forms the hexagonal Sm₃Mg₁₃Zn₃₀‐type structure also known as μ‐MgZnRE or S phase. A small phase width (x = 0–0.67; y = 0–0.58) is due to the slightly variable Ca or Zn content of the two Mg positions. The structure is described as an intergrowth of the hexagonal MgZn₂ Laves phase and the CaZn₂ structure (KHg₂‐type). All compounds exhibit strong Zn–Zn and polar Mg–Zn covalent bonds, which are visible in the calculated electron density maps. Their structures are thus herein described using the full space tilings of [Zn₄] and [MgZn₃] tetrahedra, which are fused to polyanions consisting of tetrahedra stars, icosahedra segments etc. and the large (CN = 18–22) Ca cation coordination polyhedra. Pseudo bandgaps apparent in the tDOS are compatible with the narrow v.e./M ranges observed for other isotypic members of the three structure types.     

Three polymorphs (α, β, and δ) of d‐mannitol at 100 K

In the monoclinic δ polymorph of d ‐mannitol, C₆H₁₄O₆, both the mol­ecule and the packing have approximate twofold rotational symmetry. The P2₁ structure thus approximates space group C222₁, and the α′ polymorph, previously reported in that space group, is almost certainly identical to the δ polymorph. However, torsion angles along the main backbone of the mol­ecule deviate from twofold symmetry by as much as 7.4 (3)° and the hydrogen‐bonding pattern does not conform to the higher symmetry. The α polymorph reported here is identical to the previously reported κ polymorph, and the low‐temperature structure of the β polymorph agrees well with previously reported room‐temperature determinations. The range of C—O bond lengths over the three polymorphs is 1.428 (2)–1.437 (4) Å, and the range of C—C distances is 1.515 (4)–1.5406 (19) Å. The δ polymorph has the highest density of the three, both at room temperature and at 100 K.     

The Crystal Structures of the High‐temperature Phases of Ag4Mn3O8

Two endothermic, reversible structural phase transitions of first order have been observed in Ag₄Mn₃O₈ by means of in‐situ powder diffraction and by differential scanning calorimetry. At a temperature of T = 477 K, Ag₄Mn₃O₈ undergoes a structural phase transition from the room temperature phase in space group P3₁21 to a phase in space group R32, and at T = 689 K a second phase transition to a structure in space group P4₃32 occurs. Whilst the Mn₃O₈ framework does not change significantly upon heating, rearrangements of the silver atoms, located in the cavities of the framework, were found to be the driving force behind the transitions. The structural changes with increasing temperature proceed along a path of minimal group‐supergroup relations between the respective space groups.     

Twinning by merohedry in cyclo­hexanone oxime: a revised structure

The crystal structure of cyclo­hexanone oxime, C₆H₁₁NO, was reported as severely disordered in the trigonal non‐centrosymmetric space group P3 [Olivato, Ribeiro, Zukerman‐Schpector & Bombieri (2001). Acta Cryst. B 57 , 705–713]. Re‐investigation of the crystal structure as twinned by merohedry in the trigonal centrosymmetric space group , with a twofold rotation about [001] as twin law, resulted in a well ordered structure and low R values. The asymmetric unit contains three independent mol­ecules, existing as a hydrogen‐bonded trimer, having an (9) graph set.     

A low‐temperature polymorph of m‐quinquephenyl

A low‐temperature polymorph of 1,1′:3′,1′′:3′′,1′′′:3′′′,1′′′′‐quinquephenyl (m‐quinquephenyl), C₃₀H₂₂, crystallizes in the space group P2₁/c with two molecules in the asymmetric unit. The crystal is a three‐component nonmerohedral twin. A previously reported room‐temperature polymorph [Rabideau, Sygula, Dhar & Fronczek (1993). Chem. Commun. pp. 1795–1797] also crystallizes with two molecules in the asymmetric unit in the space group P. The unit‐cell volume for the low‐temperature polymorph is 4120.5 (4) ų, almost twice that of the room‐temperature polymorph which is 2102.3 (6) ų. The molecules in both structures adopt a U‐shaped conformation with similar geometric parameters. The structural packing is similar in both compounds, with the molecules lying in layers which stack perpendicular to the longest unit‐cell axis. The molecules pack alternately in the layers and in the stacked columns. In both polymorphs, the only interactions between the molecules which can stabilize the packing are very weak C—H...π interactions.     

Lithium vanadate Li3+xV6O13 at low temperature

The structure of Li_3+xV₆O₁₃ [x = 0.24 (3)] at 95 K has been solved and refined using single‐crystal X‐ray diffraction. The refined lithium content corresponds to two fully occupied Li sites and one partially occupied Li site. A doubling of the c axis is observed upon cooling from room temperature, and this change is associated with shifts of the V atoms. The resulting space group is C2/c. The Li disorder present in the Li₃V₆O₁₃ phase at room temperature is also observed in the low‐temperature phase reported here.     

Monoclinic‐to‐orthorhombic phase transition of the hexamethylenetetramine–2‐methylbenzoic acid (1/2) cocrystal with temperature‐dependent dynamic molecular disorder

As a function of temperature, the hexamethylenetetramine–2‐methylbenzoic acid (1/2) cocrystal, C₆H₁₂N₄·2C₈H₈O₂, undergoes a reversible structural phase transition. The orthorhombic high‐temperature phase in the space group Pccn has been studied in the temperature range between 165 and 300 K. At 164 K, a t₂ phase transition to the monoclinic subgroup P2₁/c space group occurs; the resulting twinned low‐temperature phase was investigated in the temperature range between 164 and 100 K. The domains in the pseudomerohedral twin are related by a twofold rotation corresponding to the matrix (100/00/00). Systematic absence violations represent a sensitive criterium for the decision about the correct space‐group assignment at each temperature. The fractional volume contributions of the minor twin domain in the low‐temperature phase increases in the order 0.259 (2) → 0.318 (2) → 0.336 (2) → 0.341 (3) as the temperature increases in the order 150 → 160 → 163 → 164 K. The transformation occurs between the nonpolar point group mmm and the nonpolar point group 2/m, and corresponds to a ferroelastic transition or to a t₂ structural phase transition. The asymmetric unit of the low‐temperature phase consists of two hexamethylenetetramine molecules and four molecules of 2‐methylbenzoic acid; it is smaller by a factor of 2 in the high‐temperature phase and contains two half molecules of hexamethylenetetramine, which sit across twofold axes, and two molecules of the organic acid. In both phases, the hexamethylenetetramine residue and two benzoic acid molecules form a three‐molecule aggregate; the low‐temperature phase contains two of these aggregates in general positions, whereas they are situated on a crystallographic twofold axis in the high‐temperature phase. In both phases, one of these three‐molecule aggregates is disordered. For this disordered unit, the ratio between the major and minor conformer increases upon cooling from 0.567 (7):0.433 (7) at 170 K via 0.674 (6):0.326 (6) and 0.808 (5):0.192 (5) at 160 K to 0.803 (6):0.197 (6) and 0.900 (4):0.100 (4) at 150 K, indicating temperature‐dependent dynamic molecular disorder. Even upon further cooling to 100 K, the disorder is retained in principle, albeit with very low site occupancies for the minor conformer.     

Ferroelastic n‐propyl­ammonium dihydrogenphosphate

n‐Propyl­ammonium di­hydrogenphosphate, C₃H₇NH₃⁺·­H₂PO₄^?, crystals are ferroelastic at room temperature. The phase transition into the prototypic phase takes place at approximately 378 K. All atoms except two H atoms are linked by the lost symmetry operations derived from the prototypic space group P2/b2₁/n2₁/a. Each of these two different H atoms is involved in an asymmetric hydrogen bond between an oxy­gen pair. Ferroelastic switching is concomitant with jumps of these H atoms from the donor to the acceptor O atoms. The compound belongs to the structural family of n‐alkyl­ammonium di­hydrogenphosphate and in particular to the structure type of pentyl­ammonium di­hydrogenphosphate, which differs by localization of alternating layers from the rest of the known alkyl­ammonium di­hydrogenphosphates. The crystal was slightly twinned; the proportion of the minor domain was approximately 3.5%.     

Structural phase transitions in BaV6O11

BaV₆O₁₁ was synthesized under high pressures and crystallizes in a structure closely related to magnetoplumbite. [V(1)O₆]-octahedra share common edges and form a Kagomé lattice normal to the hexagonal [0 0 1] direction. The layers are connected in the direction of c via trigonal [V(3)O₅]-bipyramids and [V(2)O₆]-octahedra, which share common faces. The Ba-atoms are incorporated into cavities of the vanadium oxide framework and are coordinated by 12 oxygen atoms in the shape of a dodecahedron.Three magnetic anomalies at approximately 250, 115 and 75 K were detected in this compound. All of them are accompanied by anomalies in the specific heat measurement. To characterize possible structural transitions and determine the response of the structure to the magnetic anomalies, single crystal X-ray diffraction studies were carried out in the temperature range from 293 to 80 K. At 250 K the compound undergoes a structural phase transition. The space group above the transition temperature is P6₃/mmc, at lower temperature the symmetry reduces to P6₃mc. For the refinements in P6₃mc an inversion twin model was used, this way accounting for the loss of the center of symmetry. The structural phase transition is characterized by a small displacement of the V(1)-atom (forming the Kagomé lattice) out of its central position in the octahedra. As a consequence part of the octahedral edges/angles are increased, while the opposite ones are decreased. One limiting surface of the octahedral sheet is corrugated, while the other one is smoothened with respect to the high-temperature structure. This deformation of the octahedral sheets leads to the corresponding geometrical changes in the other coordination polyhedra.The structural response to the magnetic anomaly at 115 K is weak and mainly observable in the geometric parameters concerning the [V(1)O₆]-octahedra and [V(3)O₅]-bipyramids. This may serve as a first indication that the corresponding central atoms play an important role in the mechanism of the magnetic phase transition.     

Structural and magnetic properties of the quaternary oxides Ba6Ln2Fe4O15 (Ln=Pr and Nd)

The crystal structures and magnetic properties of the quaternary lanthanide oxides Ba₆Ln₂Fe₄O₁₅ (Ln=Pr and Nd) are reported. They crystallize in a hexagonal structure with space group P6₃mc and have the “Fe₄O₁₅ cluster” consisting of one FeO₆ octahedron and three FeO₄ tetrahedra. Measurements of the magnetic susceptibility, specific heat, and powder neutron diffraction reveal that this cluster behaves as a spin tetramer with a ferrimagnetic ground state of S_T=5 even at room temperature. The cluster moments show a long-range antiferromagnetic ordering at 23.2 K (Ln=Pr) and 17.8 K (Nd), and the magnetic moments of the Ln³⁺ ions also order cooperatively. By applying the magnetic field (∼2 T), this antiferromagnetic ordering of the clusters changes to a ferromagnetic one. This result indicates that there exists a competition in the magnetic interaction between the clusters.     

Synthesis,Structure, and Properties of SrC(NH)3, a Nitrogen‐Based Carbonate Analogue with the Trinacria Motif

Strontium guanidinate, SrC(NH)₃, the first compound with a doubly deprotonated guanidine unit, was synthesized from strontium and guanidine in liquid ammonia and characterized by X‐ray and neutron diffraction, IR spectroscopy, and density‐functional theory including harmonic phonon calculations. The compound crystallizes in the hexagonal space group P6₃/m, constitutes the nitrogen analogue of strontium carbonate, SrCO₃, and its structure follows a layered motif between Sr²⁺ ions and complex anions of the type C(NH)₃^2?; the anions adopt the peculiar trinacria shape. A comparison of theoretical phonons with experimental IR bands as well as quantum‐chemical bonding analyses yield a first insight into bonding and packing of the formerly unknown anion in the crystal.