Cooperative Jahn-Teller effect in NdMn1-xFexO3+δ (0 ≤ x ≤ 0.2)

Abstract

NdMn1-xFexO2.90(5) (0 ≤ x ≤ 0.2) compounds undergo first order reversible orthorhombic-orthorhombic structural phase transition from a high temperature O phase to low temperature O′ phase after heat treatment in Ar. This transition takes place in wide temperature range between the Jahn Teller (JT) temperature TJT and the onset of transition to orbital ordered state at T∗. Iron shifts the transition to lower temperatures TJT = 1230(5) K, 1190(5) K, 1160(5) K and T∗ = 1035(5) K, 985(5) K, 950(5) K for x = 0; 0.1; and 0.2. Both O phase and O’ phase coexist in short temperature range above T∗ and cooperative Jahn Teller distortions (CJTDs) persist even in O phase for x = 0 and 0.1 in vicinity of T∗.Oxygen is being built into crystal lattice for heat treatment in air at elevated temperatures. As a consequence the composition changes to NdMn1-xFexO3, the CJTDs vanish from O phase. The increase of oxygen in samples results in the reduction of anomalies at the phase transition and their shift to lower temperatures (TJT = 1106(5) K and T∗ = 1045(5) K for x = 0). The transformation from O phase to O’ phase is not connected with strong anomaly anymore for x = 0.1 and 0.2. The CJTDs are present below 750 K on all samples, which are temperatures from about 350 K to 250 K smaller in comparison with Ar atmosphere. Subsequent thermal cycling in air leads to phase separation into two phases for compounds with higher concentration of iron.Analysis of our results revealed that the O to O′ phase transitions are much stronger for samples in oxygen deficient state where oxygen vacancy clusters can be created at relevant temperature. We suppose that oxygen vacancy clusters are responsible for appearance of strong anomalies at the O to O′ phase transition and for easier formation of CJTDs over the volume of samples. Conditions for formation of CJTDs as well number of JT active ions in the sample are main factors which affect the O to O’ phase transition.