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  1. Abstract We use theory and first-principles calculations to investigate how structural changes induced by ultrafast optical excitation of infrared-active phonons change with hydrostatic pressure in LaAlO 3 . Our calculations show that the observed structural changes are sensitive to pressure, with the largest changes occurring at pressures near the boundary between the cubic perovskite and rhombohedral phases. We rationalize our findings by defining a figure of merit that depends only on intrinsic materials quantities, and show that the peak response near the phase boundary is dictated by different microscopic materials properties depending on the particular phonon mode being excited. Our work demonstrates how it is possible to systematically identify materials that may exhibit particularly large changes in structure and properties due to optical excitation of infrared-active phonons.
  2. Solid state compounds which exhibit non-centrosymmetric crystal structures are of great interest due to the physical properties they can exhibit. The ‘hybrid improper’ mechanism – in which two non-polar distortion modes couple to, and stabilize, a further polar distortion mode, yielding an acentric crystal structure – offers opportunities to prepare a range of novel non-centrosymmetric solids, but examples of compounds exhibiting acentric crystal structures stabilized by this mechanism are still relatively rare. Here we describe a series of bismuth-containing layered perovskite oxide phases, RbBiNb 2 O 7 , LiBiNb 2 O 7 and NaBiNb 2 O 7 , which have structural frameworks compatible with hybrid-improper ferroelectricity, but also contain Bi 3+ cations which are often observed to stabilize acentric crystal structures due to their 6s 2 electronic configurations. Neutron powder diffraction analysis reveals that RbBiNb 2 O 7 and LiBiNb 2 O 7 adopt polar crystal structures (space groups I 2 cm and B 2 cm respectively), compatible with stabilization by a trilinear coupling of non-polar and polar modes. The Bi 3+ cations present are observed to enhance the magnitude of the polar distortions of these phases, but are not the primary driver for the acentric structure, as evidenced bymore »the observation that replacing the Bi 3+ cations with Nd 3+ cations does not change the structural symmetry of the compounds. In contrast the non-centrosymmetric, but non-polar structure of NaBiNb 2 O 7 (space group P 2 1 2 1 2 1 ) differs significantly from the centrosymmetric structure of NaNdNb 2 O 7 , which is attributed to a second-order Jahn-Teller distortion associated with the presence of the Bi 3+ cations.« less