Description: Rb 4 O 6 displays mixed valency on the anionic lattice and an open p-shell electronic configuration. At ambient conditions, it exists in a disordered ground state structure, featuring three different potential ordering parameters related to phase transitions driven by charge, orbital and spin ordering. The possible paths of symmetry descent are analyzed by elaborating the respective group-subgroup relations between space groups, including the implications that would result from magnetic ordering. Experimentally, a lower symmetry modification was discovered and characterized by single-crystal X-ray diffraction [ I , Z = 4, a = 8.688(6), c = 10.362(7) Å, R 1 = 0.055, 787 independent reflections]. The symmetry reduction involved follows one branch of the Bärnighausen tree as predicted, I 3 d → I 2 d → I convincing evidence was obtained for Rb 4 O 6 undergoing charge and orbital ordering, along with respective structural order.

Description: Single-crystal X-ray diffraction experiments were performed for a series of inverse perovskites, M 3 Tt O ( M = Ca, Sr, Ba, Eu; Tt = tetrel element: Si, Ge, Sn, Pb) in the temperature range 500–50 K. For Tt = Sn, Pb, they crystallize as an `ideal' perovskite-type structure ( , cP 5); however, all of them show distinct anisotropies of the displacement ellipsoids of the M atoms at room temperature. This behavior vanishes on cooling for M = Ca, Sr, Eu, and the structures can be regarded as `ideal' cubic perovskites at 50 K. The anisotropies of the displacement ellipsoids are much more enhanced in the case of the Ba compounds. Finally, their structures undergo a phase transition at ∼ 150 K. They change from cubic to orthorhombic ( Ibmm , oI 20) upon cooling, with slightly tilted OBa 6 octahedra, and bonding angles O—Ba—O ≃ 174° (100 K). For the larger Ba 2+ cations, the structural changes are in agreement with smaller tolerance factors ( t ) as defined by Goldschmidt. Similar structural behavior is observed for Ca 3 Tt O. Smaller Tt 4− anions (Si, Ge) introduce reduced tolerance factors. Both compounds Ca 3 SiO and Ca 3 GeO with cubic structures at 500 K, change into orthorhombic ( Ibmm ) at room temperature. Whereby, Ca 3 SiO is the only representative within the M 3 Tt O family where three polymorphs can be found within the temperature range 500–50 K: – Ibmm – Pbnm . They show tiny differences in the tilting of the OCa 6 octahedra, expressed by O—Ca—O bond angles of 180° (500 K), ∼ 174° (295 K) and 170° (100 K). For larger M (Sr, Eu, Ba), together with smaller Tt (Si, Ge) atoms, pronounced tilting of the O M 6 octahedra, and bonding angles of O— M —O ≃ 160° (295 K) are observed. They crystallize in the anti -GdFeO 3 type of structure ( Pbnm , oP 20), and no phase transitions occur between 500 and 50 K. The observed phase transitions are all accompanied by multiple twinning, in terms of pseudo-merohedry or reticular pseudo-merohedry.

Description: Author(s): Denis Usvyat, Congling Yin, Gero Wälz, Claus Mühle, Martin Schütz, and Martin Jansen A first-order phase transition from the low-temperature P2 1 2 1 2 1 ( α ) phase to the high-temperature Pnma ( β ) phase of the GeF 2 crystal is studied experimentally and theoretically. GeF 2 consists of covalently bonded chains, which arrange to the three-dimensional crystal due to noncovalent intermolecula... [Phys. Rev. B 86, 054102] Published Wed Aug 01, 2012

Description: Crystal structures of partially oxidized cyanoplatinates were determined by single crystal analysis. Rb 1.75 [Pt(CN) 4 ] · 1.25H 2 O crystallizes monoclinic in the space group P 2 1 / c [ a = 1171.4(2) pm, b = 3315.3(6) pm, c = 1054.4(2) pm, β = 114.253(2)°, V = 3733.2(11) · 10 6 pm 3 , Z = 16, N total = 31567, R (F) N = 0.0858]. The compound was obtained by mixing aqueous solutions of Rb 2 [Pt(CN) 4 ] · 1.5H 2 O and Rb 2 [Pt(CN) 4 ]Br 2 . Cs 1.75 [Pt(CN) 4 ] · 0.5H 2 O crystallizes in the space group Ibam [ a = 1266.1(3) pm, b = 1347.5(3) pm, c = 580.4(1) pm, V = 990.2(3) · 10 6 pm 3 , Z = 4, N total = 5997, R (F) N = 0.0807]. Crystals of the latter were grown by slow electrochemical oxidation of Cs 2 [Pt(CN) 4 ] · H 2 O in aqueous solution using platinum wires as electrodes. In both compounds platinum is partially oxidized to Pt +2.25 and the nearly square-planar tetracyanoplatinate groups are arranged to columns in a staggered conformation. The rubidium compound features a large unit cell with nonlinear chains of platinum atoms and short alternating Pt–Pt distances of 292 and 294 pm, the cesium compound crystallizes more symmetrically with Pt–Pt distances of c /2 = 290 pm. The two compounds were also characterized by differential thermal analysis, thermogravimetry, infrared, and Raman spectroscopy.