A2B′B″O6 perovskites: A review

Group-theoretical methods are used to enumerate the structures of ordered double perovskites, A(2)BB'X(6), in which the ordering of cations B and B' into alternate octahedra is considered in combination with the ubiquitous BX(6) (or B'X(6)) octahedral tilting. The cation ordering on the B-cation site is described by the irreducible representation R(1)(+) of the Pm3;m space group of the cubic aristotype, while the octahedral tilting is mediated by irreducible representations M(3)(+) and R(4)(+). There are 12 different structures identified, and the corresponding group-subgroup relationships are displayed. Known structures are briefly reviewed.

Ordered double perovskites -- a group-theoretical analysis.

The software package SPuDS has previously been shown to accurately predict crystal structures of AMX(3) and A(1 - x)A'(x)MX(3) perovskites that have undergone octahedral tilting distortions. This paper describes the extension of this technique and its accuracy for A(2)MM'X(6) ordered double perovskites with the aristotype Fm\overline 3m cubic structure, as well as those that have undergone octahedral tilting distortions. A survey of the literature shows that roughly 70% of all ordered double perovskites undergo octahedral tilting distortions. Of the 11 distinct types of octahedral tilting that can occur in ordered perovskites, five tilt systems account for approximately 97% of the reported structures. SPuDS can calculate structures for the five dominant tilt systems, Fm\overline 3m (a(0)a(0)a(0)), I4/m (a(0)a(0)c(-)), R\overline 3 (a(-)a(-)a(-)), I2/m (a(0)b(-)b(-)) and P2(1)/n (a(-)a(-)b(+)), as well as two additional tilt systems, Pn\overline 3 (a(+)a(+)a(+)) and P4/mnc (a(0)a(0)c(+)). Comparison with reported crystal structures shows that SPuDS is quite accurate at predicting distortions driven by octahedral tilting. The favored modes of octahedral tilting in ordered double perovskites are compared and contrasted with those in AMX(3) perovskites. Unit-cell pseudosymmetry in Sr- and Ca-containing double perovskites is also examined. Experimentally, Sr(2)MM'O(6) compounds show a much stronger tendency toward pseudosymmetry than do Ca(2)MM'O(6) compounds with similar tolerance factors.

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Structure prediction of ordered and disordered multiple octahedral cation perovskites using SPuDS.

Local and cooperative Jahn-Teller interactions in model structures

A new quaternary layered transition-metal oxide, Na2Cu2TeO6, has been synthesized under air using stoichiometric (with respect to the cationic elements) mixtures of Na2CO3, CuO, and TeO2. Na2Cu2TeO6 crystallizes in the monoclinic space group C2/m with a = 5.7059(6) A, b = 8.6751(9) A, c = 5.9380(6) A, β = 113.740(2)°, V = 269.05(5) A3, and Z = 2, as determined by single-crystal X-ray diffraction. The structure is composed of [Cu2TeO6] layers with the Na atoms located in the octahedral voids between the layers. Na2Cu2TeO6 is a green nonmetallic compound, in agreement with the electronic structure calculation and electrical resistance measurement. The magnetic susceptibility shows Curie−Weiss behavior between 300 and 600 K with an effective moment of 1.85(2) μB/CuII and ϑc = −87(6) K. A broad maximum at 160 K is interpreted as arising from short-range one-dimensional antiferromagnetic correlations. With the aid of the technique of magnetic dimers, the short-range order was analyzed in terms of an alternatin...

Synthesis, structure, and magnetic properties of the layered copper(II) oxide Na2Cu2TeO6.

Es wird uber Neutronenbeugungsstrukturanalysen an den Kristallpulvern von Verbindungen Ba2CuWO6 und Sr2CuBVIO6 (BVI: W, Te) sowie zweier mit Zn2+ auf der Cu2+-Position verdunnter Mischkristalle berichtet, die in Jahn-Teller-verzerrten Varianten des Elpasolith-Typs kristallisieren. Die CuO6-Oktaeder sind betrachtlich tetragonal geweitet, mit Differenzen zwischen langen und kurzen Abstanden von etwa 0,4 A. Aus den Abstands- zusammen mit den ligandenfeldspektroskopischen Daten werden die σ- und π-Inkremente der CuO-Bindungen (d-Anteile) im Rahmen des „Angular-Overlap”-Modells berechnet. Danach induziert WVI signifikant starkere π-Bindungseffekte zwischen Kupfer und Sauerstoff als TeVI, wahrend die σ-Bindungsanteile vergleichbar stark sind.



The Crystal Structure of Cu2+-containing Oxidic Elpasolites. Neutron Diffraction Studies of the Crystal Powders



A neutron diffraction study of compounds Ba2CuWO6, Sr2CuBVIO6 (BVI: W, Te) and two related mixed crystals as well [powder data], which crystallise in Jahn-Teller distorted modifications of the elpasolite structure, yielded considerably tetragonally elongated CuO6 octahedra with differences between long and short distances of about 0.4 A. From these distances and the ligand field parameters the σ and π contributions of the CuO bonds were calculated, using the angular overlap model. Obviously WVI induces a significantly stronger π-overlap between copper and fluorine than TeVI, while the σ-overlap is of comparable strength for the d0 and d10 ions.

H. Weitzel

Papers

Die Kristallstrukturen Cu2+‐haltiger oxidischer Elpasolithe – Neutronenbeugungsuntersuchungen an den Kristallpulvern