Polymorphism in Zintl Phases ACd₄Pn₃: Modulated Structures of NaCd₄Pn₃ with Pn = P, As

Abstract

NaCd4P3 and NaCd4As3 were synthesized via short-way transport using the corresponding elements and CdI2 as mineralizer. At room temperature, the two β-polymorphs adopt the RbCd4As3 structure type which has been recently reported for alkali metal (A)-d(10) transition metal (T)-pnictides (Pn). The title compounds crystallize rhombohedrally in space group R3̅m at room temperature and show reversible phase transitions to incommensurately modulated α-polymorphs at lower temperatures. The low-temperature phases are monoclinic and can be described in space group Cm(α0γ)s with q vectors of q = (-0.04,0,0.34) for α-NaCd4P3 and q1 = (-0.02,0,0.34) for α-NaCd4As3. Thermal properties, Raman spectroscopy, and electronic structures have been determined. Both compounds are Zintl phases with band gaps of 1.05 eV for β-NaCd4P3 and ∼0.4 eV for β-NaCd4As3.

Ternary ACd4P3 (A = Na, K) Nanostructures via a Hydride Solution-Phase Route

“…Our results are in agreement with previous reports for this system. These show that the R3̅ m phase converts to a modulated Cm phase reversibly at ∼260 K. 18 Because the calculations are a 0 K approximation, we would expect that the Cm phase would be the lowest in energy and R3̅ m would be close in energy. As previously reported, the density of states calculation for R3̅ m and Cm show a small band gap (see the Supporting Information).…”

“…As mentioned above, several polymorphs of ACd 4 Pn 3 phases exist (A = Na, K, Rb) (Pn = P, As). 18 We used density functional theory (DFT) calculations to determine the thermodynamically stable phase for NaCd 4 P 3 . We considered four plausible structures: P4/mmm, P6 3 /mmc, Cm, and R3̅ m. The P4/mmm structure was determined for CsZn 4 P 3 and RbZn 4 P 3 , whereas the P6 3 /mmc structure was determined for NaZn 4 Sb 3 .…”

“…The R3̅ m structure is reported as the "β phase", 18 whereas the Cm "α phase" was reported as a modulated structure with Cd responsible for the modulation. 18 An approximation for the modulated structure was used for the computations (Figure 8 and Table 2). From the calculated total energy of the three structures, Cm exhibits the lowest energy, with R3̅ m being only 30 meV higher in energy.…”

“…KMn 4 As 3 ), tetragonal P4/ mmm (KCu 4 S 3 -type, includes (K/Rb/Cs)Zn 4 As 3 , Cs(Cd/ Mn) 4 As 3 , and RbMn 4 As 3 ), or rhombohedral R3̅ m (NaZn 4 As 3 and (Na/K/Rb)Cd 4 As 3 ) structures, and are studied for their magnetic and thermoelectric behavior. 16,19,20 Interestingly, NaCd 4 P 3 undergoes a phase transition from R3̅ m to a modulated Cm(α0γ)s structure at ∼260 K. 18 With such rich structural diversity and competing binary phases, it can be challenging to synthesize these materials in a phase-pure manner.…”

“…Pnictide compounds continue to attract significant attention because of their rich structural chemistry and because of their unique optoelectronic and catalytic properties. − Binary pnictides adopt a variety of structures with complex pnictogen motifs including tubular fragments, layers, clusters, and cages . Heavily studied ternary pnictide phases include those from the CaAl 2 Si 2 and ThCr 2 Si 2 type structures that exhibit potential in superconductivity, − thermoelectric, ,,− and optical applications. , Recently reported I–II 4 –V 3 compounds (I = alkali metal; II = divalent transition metal; V = pnictide) adopt a variety of related crystal structures (Figure ), all of which share structural similarities to CaAl 2 Si 2 and ThCr 2 Si 2 type structures. − These Zintl phases comprise edge- and corner-sharing II–V 4 tetrahedra making up polyanionic layers. The polyanionic layers are charge-balanced by intercalated I + cations (I = Na, K, Rb, or Cs).…”