Pressure-induced reconstructive phase transition inCd3As2

Abstract

Cadmium arsenide (Cd 3 As 2) hosts massless Dirac electrons in its ambient-condition tetragonal phase. We report x-ray diffraction and electrical resistivity measurements of Cd 3 As 2 upon cycling pressure beyond the critical pressure of the tetragonal phase and back to ambient conditions. We find that, at room temperature, the transition between the low-and high-pressure phases results in large microstrain and reduced crystallite size, both on rising and falling pressure. This leads to nonreversible electronic properties, including self-doping associated with defects and a reduction of the electron mobility by an order of magnitude due to increased scattering. This paper indicates that the structural transformation is sluggish and shows a sizable hysteresis of over 1 GPa. Therefore, we conclude that the transition is first-order reconstructive, with chemical bonds being broken and rearranged in the high-pressure phase. Using the diffraction measurements, we demonstrate that annealing at ∼200 • C greatly improves the crystallinity of the high-pressure phase. We show that its Bragg peaks can be indexed as a primitive orthorhombic lattice with a HP ≈ 8.68 Å, b HP ≈ 17.15 Å, and c HP ≈ 18.58 Å. The diffraction study indicates that, during the structural transformation, a new phase with another primitive orthorhombic structure may also be stabilized by deviatoric stress, providing an additional venue for tuning the unconventional electronic states in Cd 3 As 2 .

Unusual thermodynamics of low-energy phonons in the Dirac semimetal Cd3As2

“…Likewise, at T ∼ 100 K, strong frequency reduction of several optical phonons has been observed by Raman scattering, [12] and this temperature has been regarded as a characteristic energy scale of interband scattering in the Dirac states coupling to low-energy optical phonons. In line with the negative values of γ(T ) that indicate lattice instability at low temperatures, the tetragonal metallic phase of Cd 3 As 2 is indeed rather unstable and changes to a semiconducting monoclinic phase at a critical pressure p c ≈ 2.3 GPa, [14,16] as has been mentioned above. Different from the general expectation that pressure drives an insulator or a semiconductor to a metallic phase due to band broadening, the opposite trend observed in Cd 3 As 2 indicates that the Dirac bands might play an important role in the structural instability, as inferred from our thermal expansion measurements.…”

“…The bulk modulus K ≈ 57.8 GPa at 200 K is very close to that (K = 54 GPa) estimated from the lattice constants as a function of pressure. [14] Both C L (T ) and C T (T ) reveal no apparent anomaly and are characterized by smooth hardening upon cooling, as expected for typical solids. Nevertheless, we'd like to point out a weak feature that may be relevant to the Dirac states, i.e., an apparently damped harden-106501-3 ing of C T (T ) at T < 100 K. That is, C T (T ) tends to level off below 100 K and this behavior is almost absent in C L (T ).…”

“…From a crystallographic point of view, Cd 3 As 2 at ambient conditions crystalizing in the tetragonally distorted antifluorite structure (space group I4 1 /acd) that hosts topological Dirac bands is located close to a lattice instability. Upon heating to only about 220 • C, it transforms to a Zn 3 As 2 -type structure with space group (P4 2 /nbc), with at least two more structural phase transitions taking place at higher temperatures, [13,14] Alternatively, application of pressure causes a couple of structural phase transitions as well, starting from the one at a relatively low pressure of ∼ 2.3 GPa. [14][15][16] Given the complex lattice instability as introduced above, an in-depth investigation on this compound by a comprehensive set of thermodynamic probes appears to be essential in characterizing the lattice dynamics and, more importantly, its potential interaction with Dirac electrons.…”

Carbon Dots/Iron Oxide Nanoparticles with Tuneable Composition and Properties

“…The latter was assessed from PXRD pattern of high-purity (6N) germanium powder with controlled grain size. Thus, after subtracting the instrumental broadening, the volume averaged size of crystallites was calculated from refined X values in degrees using the formula: D V = 360 K S λ/(π2X), where λ is the wavelength of the radiation used for the diffraction study, and K S denoting the Scherrer constant was assumed to be 1 [47], providing lower bounds for mean sizes of diffracting grains as other contributions to peak broadening (e.g., due to microstrain) were not considered.…”