Coexistence of metallic and nonmetallic properties in the pyrochlore Lu2Rh2O7

Abstract

Transition metal oxides of the 4d and 5d block have recently become the targets of materials discovery, largely due to their strong spin-orbit coupling that can generate exotic magnetic and electronic states. Here we report the high pressure synthesis of Lu2Rh2O7, a new cubic pyrochlore oxide based on 4d 5 Rh 4+ and characterizations via thermodynamic, electrical transport, and muon spin relaxation measurements. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetic contribution, while heat capacity shows an enhanced Sommerfeld coefficient, γ = 21.8(1) mJ/mol-Rh K 2 . Muon spin relaxation measurements confirm that Lu2Rh2O7 remains paramagnetic down to 2 K. Taken in combination, these three measurements suggest that Lu2Rh2O7 is a correlated paramagnetic metal with a Wilson ratio of RW = 2.5. However, electric transport measurements present a striking contradiction as the resistivity of Lu2Rh2O7 is observed to monotonically increase with decreasing temperature, indicative of a nonmetallic state. Furthermore, although the magnitude of the resistivity is that of a semiconductor, the temperature dependence does not obey any conventional form. Thus, we propose that Lu2Rh2O7 may belong to the same novel class of non-Fermi liquids as the nonmetallic metal FeCrAs.

High-Pressure Routes to New Pyrochlores and Novel Magnetism

“…Ever more accessible higher pressures (greater than 20 GPa) and temperatures have led to new phases, such as the recently discovered silicon pyrochlore phases (Si can have octahedral symmetry under high pressure) [57]. High-pressure techniques can also be used to stabilize certain oxidation states, such as Rh 4+ and Pt 4+ [58]. Here we highlight recent work on the planate pyrochlores.…”

Honeycomb lattice iridate on the verge of Mott-collapse

“…The nonmetallic metal state was first observed in FeCrAs [47,48]. In addition, such a nonmetallic metal behavior has recently been observed in some oxides, for example, Lu2Rh2O7 [49] and Cu3LiRu2O6 [50]. Therefore, a similar weak localization state may be realized in (La,Na)IrO3 near the Mott transition.…”

Characterisation of Pb2Rh2O7 and Y2Rh2O7: an unusual case of pyrochlore stabilisation under high pressure, high temperature synthesis conditions

“…Large Wilson ratios have been reported for other Rh 4+ containing compounds, such as the 4H and 6H forms of BaRhO 3 as well as the related Rh 4+ pyrochlore Lu 2 Rh 2 O 7 ( R w = 1.89, 2.69 and 2.5 respectively). 38,78 In all these compounds the Rh 4+ cations form a geometrically frustrated lattice, being either a layered 2D Kagome lattice in the case of the BaRhO 3 hexagonal perovskites or a 3D tetrahedral arrangement in the pyrochlores. A large Wilson ratio and electron localisation are consistent with the temperature dependence of electrical resistivity shown in Fig.…”

“…An unusual temperature dependence of resistivity is observed, similar to the metallic nonmetal Lu 2 Rh 2 O 7 . 78 Further investigation of Rh 4+ containing materials, which have been shown to be reliably synthesised under HPHT conditions, is warranted in order to better understand these competing electron localisation and delocalisation effects.…”