2 - Crystal growth of magnetic pyrochlore oxides and their structure-property correlations

Abstract

The oxides crystallizing with the pyrochlore A2B2O7 structure, where A and B cations form interpenetrating networks of corner-linked tetrahedra, constitute one of the most interesting classes of materials, with wide-ranging magnetic, magnetotransport, and topological properties that are of interest not only in the domain of basic science but also in various emerging applied areas, including energy and data storage. This chapter focuses on the (A3+, B4+) class of pyrochlore oxides, where B is a main-block transition metal ion and A is a lanthanide ion. After a brief review of the crystal growth, covering various techniques typically employed for growing pyrochlore oxides, the structure–property correlations are presented in great details. The chapter examines three representative families of pyrochlore oxides, namely the titanates and zirconates A2M2O7 (M=Ti, Zr), the molybdates A2Mo2O7, and the iridates A2Ir2O7, representing the 3d, 4d, and 5d transition metal ions at the B-site. In A2M2O7, where M = Ti4+ or Zr4+ with its empty d orbitals is nonmagnetic, intriguing magnetic ground states emerge due to competing interactions between localized rare-earth moments subject to strong geometrical frustration. In A2Mo2O7, where Mo4+ also carries a magnetic moment, the physical properties are governed by the itinerant 4d electrons and the interplay of 4d-4f interactions over the geometrically frustrated pyrochlore lattice. In A2Ir2O7, on the other hand, a substantial spin-orbit coupling induces nontrivial topological phases, including quadratic band touching and linear band crossing with Weyl nodes near the Fermi energy. These have been a subject of considerable interest in recent years. The primary focus throughout the chapter is on reviewing the experimental and theoretical progress in all three families in order to highlight the structure–property correlations in these pyrochlores. The emphasis has been placed of reviewing the properties emerging due to structural aberrations, including, stuffing (or antistuffing), site dilutions, oxygen off-stoichiometry and doping in these pyrochlores. The chapter ends by outlining future perspectives for the study of pyrochlore oxides.