AUTHOR=Mishra Swarnamayee , Yanda Premakumar , Bhat Shrikant , Etter Martin , Sundaresan A. 

TITLE=High-pressure synthesis and magnetic properties of tetragonal R2BaCuO5 (R = Sm and Eu)

JOURNAL=Frontiers in Chemistry

VOLUME=11

YEAR=2023

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2023.1166475

DOI=10.3389/fchem.2023.1166475

ISSN=2296-2646

ABSTRACT=<p>We report the experimental discovery of a new structural phase of well-known orthorhombic <italic>R</italic><sub>2</sub>BaCuO<sub>5</sub> (<italic>R</italic> = Sm and Eu), exhibiting a tetragonal crystal structure with space group <inline-formula id="inf1"><mml:math id="m1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>P</mml:mi><mml:mn>4</mml:mn><mml:mo>∕</mml:mo><mml:mi>m</mml:mi><mml:mi>b</mml:mi><mml:mi>m</mml:mi></mml:mrow></mml:math></inline-formula>. The high-pressure tetragonal phase is isostructural with the brown phase <italic>R</italic><sub>2</sub>BaCuO<sub>5</sub> (<italic>R</italic> = La, Pr, and Nd). In this structure, the Cu ions form an isolated square planar environment, contrary to the orthorhombic phase, where the Cu ions are located in a distorted square pyramid. Magnetization and specific heat measurements reveal the long-range antiferromagnetic order of the Cu<sup>2+</sup> and/or Sm<sup>3+</sup> moments for the Sm-sample, with the magnetic specific heat accounting for only 35% of the magnetic entropy. Interestingly, the Eu-sample remains paramagnetic down to the lowest temperature. The high Curie-Weiss temperature of −140 K and magnetic entropy of 3% of the expected value indicates that the system is highly frustrated. We estimated the isothermal entropy change and investigated the magnetocaloric effect for Eu<sub>2</sub>BaCuO<sub>5</sub>, and the maximum entropy change detected at a field of 70 kOe at 3 K reaches 5.6 J kg<sup>-1</sup>K<sup>−1</sup>.</p>