High-Pressure Routes to New Pyrochlores and Novel Magnetism

Abstract

The pyrochlore structure (A2B2O7) has been an object of consistent study by materials scientists largely due to the stability of the cubic lattice with respect to a wide variety of chemical species on the A or B sites. The criterion for stability under ambient conditions is controlled by the ratio of these cations, which is empirically 1.36 < RA/RB < 1.71. However, under applied pressure synthesis conditions, the pyrochlore lattice is stable up to RA/RB ∼ 2.30, opening up possibilities for new compounds. In this review, we will highlight recent work in exploring new rare-earth pyrochlores such as the germanates RE2Ge2O7 and platinates RE2Pt2O7. We highlight recent discoveries made in these pyrochlores such as highly correlated spin ice behavior, spin liquid ground states, and exotic magnetic ordering.

Traversing the pyrochlore stability diagram; microwave-assisted synthesis and discovery of mixed B-site Ln$_2$InSbO$_7$ family

“…Using a simple scoring algorithm, we have recalculated the limits of 72,73], the best fit, denoted by the black boundary in Fig. 2, was calculated to be [R min , R max ] of [1.46 Å, 1.82 Å], in excellent agreement with the historical estimate provided by Subramanian et al [1].…”

“…Part of the allure of the pyrochlore lattice is the chemical diversity offered by the choice of A-site and B-site elements [1,2,4,5,7,72,73,91,92]. As established previously, the community leverages the empirically-derived "radius-ratio rules" to predict and synthesize new pyrochlore materials.…”

“…For trivalent lanthanide oxide pyrochlores Ln 2 B 2 O 7 , the relationship was previously established decades ago as 1.46 ≤ R Ln 3+ /R B 4+ ≤ 1.80 [1]. More recently, the rules are often quoted as 1.36 ≤ R Ln 3+ /R B 4+ ≤ 1.71 [2,72,73]. The pyrochlore stability rules have been applied with great success in discovery of stable and metastable Ln 2 B 2 O 7 phases and continue to play a fundamental role in materials discovery.…”

“…These approaches are reminiscent of the Goldschmidt tolerance factor for perovskites [71], and aim to provide simple guidelines and chemical intuition. Within pyrochlores, the "radius-ratio rules" have been the most successful, which constrain R min ≤ R Ln 3+ /R B 4+ ≤ R max [1,2,72,73]. Despite the apparent simplicity and variations in the specifics of R min and R max , the radii-ratio rules have been applied with great suc-cess [1,2].…”

A Comparison of Order-Disorder in Several Families of Cubic Oxides

“…Though order-disorder transitions have been induced by pressure in pyrochlores, not much thermodynamic work has been carried out on them. New pyrochlores with A 2 B 2 O 7 stoichiometry have been suggested to form under high pressure ( Zhou and Wiebe, 2019 ). It will be interesting to study the energetics of pressure-induced order-disorder transformations to explore behavior under extreme conditions.…”