Rattling phenomena have been observed in materials characterized by a large cage structure but not in a simple ABO3-type perovskite because the size mismatch, if it exists, can be relieved by octahedral rotations. Here, we demonstrate that a stoichiometric perovskite oxide NaWO3, prepared under high pressure, exhibits anharmonic phonon modes associated with low-energy rattling vibrations, leading to suppressed thermal conductivity. The structural analysis and the comparison with the ideal perovskite KWO3 without rattling behavior reveal that the presence of two crystallographic Na1 (2a) and Na2 (6b) sites in NaWO3 (space group Im3̅) accompanied by three in-phase WO6 octahedral (a+a+a+) rotations generates an open space Δ ∼ 0.5 Å for the latter site, which is comparable with those of well-known cage compounds of clathrates and filled skutterudites. The observed rattling in NaWO3 is distinct from a quadruple perovskite AA′3B4O12 (A, A′: transition metals) where the A (2a) site with lower multiplicity is the rattler. The present finding offers a general guide to induce rattling of atoms in pristine ABO3 perovskites.

A high-pressure reaction yielded the fully occupied tetragonal tungsten bronze K W O (K WO ). The terminal phase shows an unusual transport property featuring slightly negative temperature-dependence in resistivity (dρ/dT<0) and a large Wilson ratio of R =3.2. Such anomalous metallic behavior possibly arises from the low-dimensional electronic structure with a van Hove singularity at the Fermi level and/or from enhanced magnetic fluctuations by geometrical frustration of the tungsten sublattice. The asymmetric nature of the tetragonal tungsten bronze K WO -K Ba WO phase diagram implies that superconductivity for x≤0.45 originates from the lattice instability because of potassium deficiency. A cubic perovskite KWO phase was also identified as a line phase-in marked contrast to Na WO and Li WO with varying quantities of x (<1). This study presents a versatile method by which the solubility limit of tungsten bronze oxides can be extended.

Compounds with the LiNbO3-type structure are important for a variety of applications, such as piezoelectric sensors, while recent attention has been paid to magnetic and electronic properties. However, all the materials reported are stoichiometric. This work reports on the high-pressure synthesis of lithium tungsten bronze LixWO3 with the LiNbO3-type structure, with a substantial non-stoichiometry (0.5 ≤ x ≤ 1). Li0.8WO3 exhibit a metallic conductivity. This phase is related to an ambient-pressure perovskite phase (0 ≤ x ≤ 0.5) by the octahedral tilting switching between a−a−a− and a+a+a+.

A high-pressure reaction yielded the fully occupied tetragonal tungsten bronze K 3 W 5 O 15 (K 0.6 WO 3 ). The terminal phase shows an unusual transport property featuring slightly negative temperature-dependence in resistivity (d1/dT < 0) and a large Wilson ratio of R W = 3.2. Such anomalous metallic behavior possibly arises from the low-dimensional electronic structure with a van Hove singularity at the Fermi level and/or from enhanced magnetic fluctuations by geometrical frustration of the tungsten sublattice. The asymmetric nature of the tetragonal tungsten bronze K x WO 3 -K 0.6Ày Ba y WO 3 phase diagram implies that superconductivity for x 0.45 originates from the lattice instability because of potassium deficiency. A cubic perovskite KWO 3 phase was also identified as a line phase-in marked contrast to Na x WO 3 and Li x WO 3 with varying quantities of x (< 1). This study presents a versatile method by which the solubility limit of tungsten bronze oxides can be extended.Since its discovery in 1823, tungsten bronze with M x WO 3 (M = alkali metal, alkali earth metal, H + , NH 4 + , and so forth) has been investigated widely. It is characterized by cornersharing WO 6 octahedra with M cations incorporated at several one-dimensional (1D) channels, leading to various structural types such as tetragonal and hexagonal tungsten bronzes (TTB and HTB, respectively; Figure 1 a; Supporting Information, Figure S1).[1] This structural (or pore-size) diversity provides a variety of novel properties that include a metal-insulator transition, [2] charge-density-wave (CDW) transition, [3] and superconductivity (SC).[4] Reversible intercalation of the M cation offers various applications ranging from secondary batteries and gas sensors to electrochromic devices. [5] The physics of the HTB system, with foreign cations incorporated up to x max = 1/3 in hexagonal 1D channels, is fairly well-understood. For example, HTB M x WO 3 (M = K, Rb) has two superconducting phases (with T c maxima at x % 1/ 3 and 0.16) separated by a CDW phase near x = 1/4 that is associated with M cation order/disorder.[6] On the contrary, the understanding of the TTB system is unsatisfactory, which is largely due to limited compositional availability: 0.2 x-(Na) 0.4, 0.38 x(K) 0.5, and x(Ba) % 0.14. No TTB compound with a complete filling at tetragonal and pentagonal 1D channels (corresponding to x max = 3/5) is known. Therefore, although SC is seen in Na x WO 3 (0.2 x 0.4), [7,8] K x WO 3 (0.38 x 0.45), [9] and Ba x WO 3 (x % 0.14), [10] the underlying mechanism behind the SC remains under debate. To explain their T c increase with reducing x, several scenarios have been proposed, including screening effect related to 2D electronic structure, [11] acoustic plasmons, [12] and lattice instability. [8,13] To detect SC in TTB, it is crucial to expand its solubility range and quantity. Herein, we show that high-pressure synthesis can expand the upper solubility range to x max = 3/5 (that is, M 3 W 5 O 15 ). K 3 W 5 O 15 (K 0.6 WO 3 ) shows an ano...

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