Structural instability and the Mott-Peierls transition in a half-metallic hollandite :K2Cr8O16

Abstract

In order to explore the driving mechanism of the concomitant metal-insulator and structural transitions in quasi-one-dimensional hollandite K 2 Cr 8 O 16 , electronic structures and phonon properties are investigated by employing the ab initio density functional theory (DFT) calculations. We have found that the imaginary phonon frequency reflecting the structural instability appears only in the DFT + U (U : Coulomb correlation) calculation, which indicates that the Coulomb correlation plays an essential role in the structural transition. The lattice displacements of the softened phonon at X explain the observed lattice distortions in K 2 Cr 8 O 16 perfectly well, suggesting the Peierls distortion vector Q of X (0, 0, 1/2). The combined study of electronic and phonon properties reveals that half-metallic K 2 Cr 8 O 16 , upon cooling, undergoes the correlation-assisted Peierls transition to become a Mott-Peierls ferromagnetic insulator at low temperature.

Electronic Structure Evolution across the Peierls Metal-Insulator Transition in a Correlated Ferromagnet

“…The calculated DOS in the metallic and insulating phases [Figs. 3(a) and 3(b), respectively] are quite similar to recent GGA þ U calculations with U ¼ 3 eV [15,17]. As is also observed in experimental VB spectra, the VB features of the calculated DOS seem very similar in the two phases, and no major changes are seen over large energy scales [Figs.…”

“…for the Cr 3þ and Cr 4þ states, the pinning of valence fluctuations suggests that the Peierls insulating phase most likely involves excitations of the type d nþ1 L 1 þ d nþ1 L 1 ↔ d nþ1 L 2 þ d nþ1 L 0 ; i.e., the charge fluctuations involve ligand holes, as was recently discussed for the nickelates[45]. Thus, while the Mott-Peierls picture describes spectral weight changes over large energy scales (about 1-2 eV), as has been discussed for the MIT in VO 2[18][19][20]46], and very recently, also for the MIT in related hollandites,K 2 V 8−x Ti x O 16[47], the present spectroscopy results with a low-energy-scale BCS-like T-dependent gap rule out a Mott-Peierls transition in K 2 Cr 8 O 16 , in contrast with such a proposition made in a very recent GGA þ U study[17]. Our results of K 2 Cr 8 O 16 are summarized in…”

“…Based on GGA þ U calculations, it was proposed that K 2 Cr 8 O 16 undergoes a quasi-1D "pure" Peierls instability of fully spin-polarized electrons at the Fermi level (E F ), giving rise to a MIT where electron correlations essentially play no role [15]. In contrast, a very recent GGA þ U study has claimed that K 2 Cr 8 O 16 exhibits a Mott-Peierls transition [17]. However, it is noted that a Mott-Peierls transition is expected to show spectral changes in the 3d density of states (DOS) over large energy scales of the order of the on-site Coulomb energy U, as is well known from the case of VO 2 [18][19][20].…”

“…We experimentally answer this question in the present work. In this context, it is also important to clarify the terminology used in the literature: A "Mott-Peierls transition" is also called a "correlation-assisted Peierls transition"; i.e., the emphasis is on the Peierls transition [17][18][19][20]. This is different from the term "Peierls-Mott transition," which is used to describe a "Peierls-assisted orbital-selective Mott transition" with the emphasis on the Mott transition [21].…”

Charge localization and ordering in A2Mn8O16 hollandite group oxides: Impact of density functional theory approaches

“…A separate GGA+U eff =3 eV based study identified a specific soft mode associated with this Peierls driven transition. 43 The situation for hollandite group manganese oxides under study here has been much less clear. Early studies of the parent α−MnO 2 showed antiferromagnetic order below 24.5 K. 44 Studies of K x Mn 8 O 16 with x near 1.5 showed a change in the activation energy associated with electrical conductivity around 200 K and changes in magnetic susceptibility at low temperature with some ambiguity as to the details of the ordering.…”