Anomalous electronic states of hollandite-type transition-metal oxides

Abstract

Abstract. We make the electronic structure calculations of transition-metal oxides with the hollandite-type crystal structure A2M8O16 using the generalized gradient approximation (GGA) in the density functional theory, where the Hubbard-type repulsive interaction is taken into account (GGA+U ). We first discuss generic electronic structures of the 3d (M =Ti, V, Cr, Mn) and 4d (M =Mo, Ru, Rh) series of the materials and then consider the origins of the metal-insulator transition observed in K2Cr8O16 and quasi-one-dimensional electron conduction observed in K2Ru8O16. We also consider in particular the observed metal-insulator transition in K2V8O16. IntroductionHollandite materials have attracted considerable attention in recent years in the field of physics of strong electron correlations. The crystal structure of hollandites resembles that of the rutile structure but the single chains of M O 6 octahedra (M =transition element) in rutiles are replaced by the double chains of the edge-shared M O 6 octahedra in hollandites, resulting in a sparse structure with large tunnels, wherein a variety of cations A can be introduced as in the chemical formula A x M 8 O 16 with 0 ≤ x ≤ 2. We point out that the material series with the 3d transitionmetal elements M = Ti, V, Cr, and Mn, which are in a mixed valent state, show a variety of unusual electronic and magnetic properties, including the metal-insulator transition, spinsinglet formation, as well as ferromagnetism and antiferromagnetism. The material series with the 4d transition-metal elements M = Mo, Ru, and Rh also show interesting physical properties, including the quasi-one-dimensional electron conduction [1].In this paper, motivated by such materials development, we make the electronic structure calculations on the series of hollandite-type transition-metal oxides to clarify their basic electronic structures. We use the generalized gradient approximation (GGA) in the density functional theory, where the Hubbard-type repulsive interaction U is taken into account (GGA+U ). We thereby examine the 3d series (M =Ti, V, Cr, Mn) as well as the 4d series (M =Mo, Ru, Rh) to discuss generic features in the electronic structure of hollandites first. Then, we in particular study the quasi-one-dimensional electron conduction observed in K [2,3] is discussed in this respect. We also study the electronic structure of K 2 V 8 O 16 to consider the origins of the observed anomalous electronic states and metal-insulator transition [4].

Control of the Iridium Oxidation State in the Hollandite Iridate Solid Solution K1–xIr4O8

“…16 Additionally, the series of 4d transition-metal oxide hollandites, including B = Mo, Ru, and Rh, also displays interesting properties, including Q-1D conduction. 19 Most interestingly, BaRu 6 O 12 shows a highly unusual temperature dependence of magnetic susceptibility and has a possible quantum phase transition near T = 2 K between a metallic state and a competing, weakly localized state, tunable by the disorder and magnetic field. 20 In this work we report the structure of KIr 4 O 8 based on single-crystal X-ray diffraction (XRD) and report its physical properties.…”

“…These compounds display various unusual physical properties, including metal–insulator transitions in KCr 4 O 8 , KV 4 O 8 , and Pb 1.6 V 8 O 16 ; spin-singlet formation in KV 4 O 8 ; ferromagnetism in KCr 4 O 8 ; and antiferromagnetism in Pb 1.6 V 8 O 16 . Additionally, the series of 4d transition-metal oxide hollandites, including B = Mo, Ru, and Rh, also displays interesting properties, including Q-1D conduction . Most interestingly, BaRu 6 O 12 shows a highly unusual temperature dependence of magnetic susceptibility and has a possible quantum phase transition near T = 2 K between a metallic state and a competing, weakly localized state, tunable by the disorder and magnetic field…”

Spin-glass behavior in KxRu4−yNiyOStimpson1, Powell2, Stenning3 et al. 2018Phys. Rev. B9010View full textAdd to dashboardCite

“…19 More recently quasi one dimensional electron conduction has been suggested for KRu4O8 from Density Functional Theory (DFT) calculations. [20][21][22] These authors further suggested that KRu4O8 could be considered to be a Tomonaga-Luttinger liquid. 22,23 The influence of structural disorder in AxRu4O8 hollandite materials has been investigated, where A is K, Rb or a mixture of Na and Rb.…”

μ+SR Study of K2Cr8O16 Under Hydrostatic Pressure

“…One-dimensional (1D) magnets have been extensively investigated during the last years as they could give an insight in the interplay between quantum and thermal fluctuations. In this respect, hollandite type materials are one of the 1D materials that have attracted a lot of interest [1][2][3]. These materials with a general chemical formula A x M 8 O 16 have a common tunnel structure constructed from the M 2 O 4 framework formed by the zigzag chains of edge-shared MO 6 octahedra and A cation at the tunnel sites, as shown in Fig.…”