First-order phase transition to a nonmagnetic ground state in nonsymmorphic NbCrP

We report the discovery of a first-order phase transition at around 125 K in NbCrP, which is a nonsymmorphic crystal with the $Pnma$ space group. From the resistivity, magnetic susceptibility, and nuclear magnetic resonance measurements using crystals made by the Sn-flux method, the high-temperature (HT) phase is characterized to be metallic with a non-negligible magnetic anisotropy. The low-temperature (LT) phase is also found to be a nonmagnetic metallic state with a crystal of lower symmetry. In the LT phase, the spin susceptibility is reduced by $\sim 30\%$ from that in the HT phase, suggesting that the phase transition is triggered by the electronic instability. The possible origin of the phase transition in NbCrP is discussed based on the electronic structure by comparing it with those in other nonsymmorphic compounds, RuP and RuAs.

Figure 1

Crystal structure of NbCrP at room temperature (TiNiSi type, space group $Pnma$). The Cr ions form the zigzag chains along the $b$ axis. All the ions occupy the equivalent sites in the $Pnma$ space group.

Figure 2

X-ray diffraction pattern of a NbCrP twin crystal at room temperature (TiNiSi type, Space group $Pnma$). The two sets of the reciprocal lattice patterns (blue and light yellow) are observed, showing that the crystal is twinned. Corresponding reciprocal lattice vectors are shown by the arrows (the view is from the $b$ axis direction).

Figure 3

Temperature dependences of (a) resistivity $\rho$ and (b) magnetic susceptibility $\chi$ of NbCrP. $\chi$ was measured under 1 T for many small crystals (similar to powdered sample). Solid arrows indicate the direction of the thermal processes. Both $\rho$ and $\chi$ data clearly show a first-order phase transition at around 100–150 K. The corrected $\chi$ by subtracting the Curie-Weiss contribution (blue dashed line) of paramagnetic impurity from observed $\chi$ is also shown by open symbols.

Figure 4

(a) Temperature dependence of ${}^{31}\mathrm{P}$ NMR spectra for the loosely packed NbCrP sample, which is most likely partially oriented, measured at $H$ = 5.05 T. (b) Temperature dependence of the ${}^{31}\mathrm{P}$ NMR Knight shift $K$ for the loosely packed powder sample. (c) $K$ vs $\chi$ plot. The solid line is the fitting result for the HT phase. The dashed lines are drawn for the LT phase assuming that $K_{\mathrm{orb}}$ is the same as that in the HT phase.

Figure 5

(a) Temperature dependence of the ${}^{31}\mathrm{P}1/T_{1}T$. (b) Temperature dependence of ${(1/T_{1}T)}^{1/2}$ and $K_{\mathrm{spin}}$ normalized by the values at 295 K.

Figure 6

(a) Temperature dependence of field-swept ${}^{31}\mathrm{P}$ NMR spectra ($f$ = 130 MHz) for oriented samples measured under magnetic fields parallel to the easy axis (red curves) and perpendicular to the easy axis (blue curves). (b) Temperature dependence of the ${}^{31}\mathrm{P}$ NMR Knight shifts for the oriented samples for the two magnetic field directions. (c) Temperature dependence of the ${}^{31}\mathrm{P}1/T_{1}T$.

Figure 7

Typical field-swept ${}^{93}\mathrm{Nb}$ NMR spectra measured using the oriented sample in the LT phase ($T$ = 4.3 K) and the HT phase ($T$ = 160 K) for (a) $H_{\left|\right|}$ and (b) $H_{\perp }$. The colored areas show the simulated results. (c) The schematic view shows the directions for principal axes of the EFG with respect to the crystalline axes obtained from the electronic band structure calculation.

Figure 8

(a) The energy dispersion and (b) the density of states of NbCrP calculated for orthorhombic TiNiSi-type structure in the $Pnma$ space group in the HT phase. The partial DOS is shown by different colors (red: Cr $3d$; green: Nb $4d$). (c) A comparison of the energy dispersions between NbCrP and RuAs near the Fermi level. The fourfold degeneracy is protected along the directions shown by the red lines, and they are slightly lifted along the directions shown by the blue lines due to the spin-orbit coupling. The nearly flat degenerate bands near $E_{\mathrm{F}}$ are shown by the green circles.

Figure 9

Fermi surface of NbCrP calculated for the orthorhombic TiNiSi-type structure in the $Pnma$ space group in the HT phase. They consist of several three-dimensional sheets.