Incommensurate–Commensurate Transition in the Geometric Ferroelectric LaTaO₄

Abstract

The layered perovskite LaTaO 4 has been synthesized to be stable in both (polar) orthorhombic and (nonpolar) monoclinic polymorphs at ambient conditions. Although the structural transition between monoclinic and orthorhombic phases has been well established, there is some controversy regarding a further, unidentified transition around 500 K. Here this is identified as an incommensurate-commensurate first-order transition between incommensurate Cmc2 1 (α00)0s0 and commensurate Cmc2 1 orthorhombic phases. Transmission electron microscopy indicates partially ordered stacking of different structural units in a, identifying the local cause for the modulation, whereas variable temperature powder neutron diffraction has shown the overall macroscopic modulation vector, q ≈ (0.456, 0, 0)-roughly a 2.2 × expansion in a, corresponding to an approximate 11a commensurate superunit cell dimension. The modulation shows a continuous temperature dependence until transitioning to the basic (commensurate) cell at T IC-C. Doping the interlayer La sites with smaller Nd cations stabilizes the incommensuration to higher temperature, suggesting the modulation is geometrically driven at the A site.

Structural phase transitions in the geometric ferroelectric LaTaO4

“…Samples synthesized in this investigation, which are monoclinic (P2 1 /c) at ambient conditions, undergo a first-order structural transition to the orthorhombic (Cmc2 1 ) phase at ∼440 K on heating but display a significant thermal hysteresis, transforming back to monoclinic at ∼300 K. Transitions between the P2 1 /c and Cmc2 1 phases are easily detected by means of dielectric spectroscopy, but this technique also reveals a second dielectric anomaly at approximately 500 K (this feature exhibits little thermal hysteresis) and is also present in samples that are already stabilized in the Cmc2 1 phase at ambient [11,12]. Until recently, the origin of this dielectric anomaly was subject to debate, but we have identified it as an incommensurate [Cmc2 1 (α00)0s0] to commensurate (Cmc2 1 ), IC-C transition [14]. The modulation of this phase was found to arise as a partial ordering of smaller subunits in the a axis, which led to an overall modulation wave vector of q = (0.456, 0, 0) at 483 K (active irrep 2 [0.456, 0, 0]).…”

“…There are two derivative structures which develop from this by different combinations of octahedral tilting as described previously and are shown in Fig. 1: (a) An orthorhombic structure with space group Cmc2 1 , which is polar and has the same unit cell dimensions as the parent Cmcm structure (but with an incommensurate repeat of ∼2.2 unit cells along the a axis, as identified in our previous study [14]), and (b) a monoclinic structure with space group P2 1 /c, which is nonpolar. As mentioned above, synthesis conditions can be adjusted to predetermine which LaTaO 4 polymorph will be stabilized at ambient conditions.…”

“…Variable temperature powder neutron diffraction (PND) data for Sample A was also collected using the high-resolution powder diffractometer (HRPD) at ISIS as described in Ref. [14]. Samples were first heated to above 523 K to transform any residual monoclinic phase to orthorhombic and data were subsequently collected on cooling from 523 K at 2 K increments to 483 K; for close observation of the IC-C phase transition; collection time for each time was 1 h (40 μAh integrated proton current to the target), except 483 and 523 K, which was 3 h (120 μAh).…”

“…The data in this region of phase coexistence is therefore fitted to a mixture of low (IC) and high (C) temperature Cmc2 1 phases each with distinct lattice parameters. The IC phase is slightly contracted in the a-c plane but with a much larger b axis as a result the modulation inhibiting the close packing of the octahedral sheets compared to that of the commensurate Cmc2 1 phase [14]. The satellite peaks associated with the IC phase are only detectable by PND and disappear with increasing temperature.…”

“…Since the structure is only modulated in the a axis the wave vector only contains an a* component, which is found to be negatively correlated with temperature. This means that modulation increases in length with increasing temperature in real space, i.e., from an average repeat of 2.179 a at 323 K to 2.193 a at 483 K. The increasing length of the real-space modulation is thought to be due to an increased ordering of these subunits as a function of temperature [14]. The existence of these smaller subunits, which are 3 × and 2.5 × expansions of the basic cell in the a axis, was hypothesized to be the result of a mechanism that rectifies an irregular La bonding environment, as seen in n = 4 phases [3].…”