Double Double to Double Perovskite Transformations in Quaternary Manganese Oxides

Abstract

Control of cation ordering in ABX3 perovskites is important to structural, physical and chemical properties. Here we show that thermal transformations of AA′BB′O6 double double perovskites, where both A and B sites have 1:1 cation order, to (A0.5A′0.5)2BB′O6 double perovskites with fully disordered A/A′ cations can be achieved under pressure in CaMnMnWO6 and SmMnMnTaO6, enabling both polymorphs of each material to be recovered. This leads to a dramatic switch of magnetic properties from ferrimagnetic order in double double perovskite CaMnMnWO6 to spin glass behaviour in the highly frustrated double perovskite polymorph. Comparison of double double and double perovskite polymorphs of other materials will enable effects of cation order and disorder on other properties such as ferroelectricity and conductivity to be explored.

Coexisting commensurate and incommensurate magnetic orders in the double double perovskite CaMnCoWO6

“…They all have a tetragonal P4 2 /n structure with columnar ordering of 10-coordinate A and 4-coordinate A' cations. A = R (rare earth) RMnMnB'O 6 series are reported for B' = Sb [18,19] and Ta, [20] and many CaMnBB'O 6 materials (B = Mn, Fe, Co, Ni for B' = Re, [21,22,23] and B/B' = Fe/Ta, [24] Cr/Sb and Fe/Sb, [25] Mn/W, [26] and Fe/Nb) [27] have also been synthesised at high temperatures and pressures. Recently P4 2 /n double double perovskites based on A' cations other than Mn 2 + have been reported in CaCuFeReO 6 [28] and CaFeFeNbO 6 .…”

“…Most of the P 4 2 / n double double perovskite materials have simple ferri‐ or ferro‐ magnetic orders commensurate with the crystal unit cell (propagation vector k =(0 0 0)), although CaMnFeTaO 6 , CaMnFeNbO 6 , and CaFeFeNbO 6 have shown spin glass ground states without long range spin ordering due to substantial cation disorder [24,27] . Two materials, CaMnMnWO 6 and SmMnMnTaO 6 , were discovered to undergo thermal transformations from AA'BB'O 6 double double perovskite structures, where both A and B sites have 1 : 1 cation order, to (A 0.5 A’ 0.5 ) 2 BB'O 6 double perovskites with fully disordered A/A’ cations under pressure [26] . For CaMnMnWO 6 this leads to a drastic change of magnetic properties from ferrimagnetic order in the double double perovskite form to spin glass behaviour in the highly frustrated double perovskite polymorph.…”

“…[24,27] Two materials, CaMnMnWO 6 and SmMnMnTaO 6 , were discovered to undergo thermal transformations from AA'BB'O 6 double double perovskite structures, where both A and B sites have 1 : 1 cation order, to (A 0.5 A' 0.5 ) 2 BB'O 6 double perovskites with fully disordered A/A' cations under pressure. [26] For CaMnMnWO 6 this leads to a drastic change of magnetic properties from ferrimagnetic order in the double double perovskite form to spin glass behaviour in the highly frustrated double perovskite polymorph. This motivated the present exploration of the analogue CaMnCoWO 6 which is discovered to form an offstoichiometric double double perovskite CaMn 0.8 Co 1.2 WO 6 with co-existing commensurate and incommensurate spin orders, the first time that such complex magnetism has been observed in this family of materials.…”

CaFeFeNbO6 – an iron-based double double perovskite

“…Many P 4 2 / n AA’BB’O 6 double double perovskites with A’ = Mn have been synthesised at 10 GPa; RMnMnB’O 6 (B’ = Sb 15,16 and Ta 17 ) and CaMnBB’O 6 (B = Mn, Fe, Co, Ni for B’ = Re, 18–20 and B/B’ = Fe/Ta, 21 Cr/Sb, Fe/Sb 22 and Mn/W 23 ); and recently the first non-Mn analogue CaCuFeReO 6 was made at 15.5 GPa. 24 The CaA’FeReO 6 (A’ = Mn, Mn 0.5 Cu 0.5 , Cu) materials are notable as ferrimagnets with Curie temperatures above 500 K and varying magnetoresistance properties.…”