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Register a new userInducing Spontaneous Electric Polarizations in Double Perovskite Iodide Superlattices for New Ferroelectric Photovoltaic Materials
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In this work, we use density functional theory calculations to demonstrate how spontaneous electric polarizations can be induced textit{via} a hybrid improper ferroelectric mechanism in iodide perovskites, a family well-known to display solar-optimal band gaps, to create new materials for photoferroic applications. We first assemble three chemically distinct ($A$$A^{prime}$)($B$$B^{prime}$)I$_6$ double perovskites using centrosymmetric $AB$I$_3$ perovskite iodides (where $A$ = Cs, Rb, K and $B$ = Sn, Ge) as building units. In each superlattice, we investigate the effects of three types of $A$- and $B$-site cation ordering schemes and three different $B$I$_6$ octahedral rotation patterns. Out of these 27 combinations, we find that 15 produce polar space groups and display spontaneous electric polarizations ranging from 0.26 to 23.33 $mu$C/cm$^2$. Furthermore, we find that a layered $A$-site/rock salt $B$-site ordering, in the presence of an $a^0a^0c^+$ rotation pattern, produces a chiral vortex-like $A$-site displacement pattern. We then investigate the effect of epitaxial strain on one of these systems, (CsRb)(SnGe)I$_6$, in layered and rock salt ordered configurations. In both phases, we find strong competition between the cation ordering schemes as well as an enhancement of the spontaneous polarization magnitude under tensile strain. Finally, using advanced functionals, we demonstrate that these compounds display low band gaps ranging from 0.2 to 1.3 eV. These results demonstrate that cation ordering and epitaxial strain are powerful ways to induce and control new functionalities in technologically-useful families of materials.
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We present measurements of conductance hysteresis on CH3NH3PbI3 perovskite thin films, performed using the double-wave method, in order to investigate the possibility of a ferroelectric response. A strong frequency dependence of the hysteresis is observed in the range of 0.1 Hz to 150 Hz, with a hysteretic charge density in excess of 1000 {mu}C/cm2 at frequencies below 0.4 Hz - a behaviour uncharacteristic of a ferroelectric response. We show that the observed hysteretic conductance, as well as the presence of a double arc in the impedance spectroscopy, can be fully explained by the migration of mobile ions under bias on a timescale of seconds. Our measurements place an upper limit of approximately 1 {mu}C/cm2 on any intrinsic frequency-independent polarisation, ruling out ferroelectricity as the main cause of current-voltage hysteresis and providing further evidence of the importance of ionic migration in modifying the efficiency of CH3NH3PbI3 devices.
We review all the published literature and show that there is no experimental evidence for homogeneous tin titanate SnTiO3 in bulk or thin-film form. Instead a combination of unrelated artefacts are easily misinterpreted. The X-ray Bragg data are contaminated by double scattering from the Si substrate, giving a strong line at the 2-theta angle exactly where perovskite SnTiO3 should appear. The strong dielectric divergence near 560K is irreversible and arises from oxygen site detrapping, accompanied by Warburg/Randles interfacial anomalies. The small (4 uC/cm2) apparent ferroelectric hysteresis remains in samples shown in pure (Sn,Ti)O2 rutile/cassiterite, in which ferroelectricity is forbidden. Only very recent German work reveals real bulk SnTiO3, but this is completely inhomogeneous, consisting of an elaborate array of stacking faults, not suitable for ferroelectric devices. Unpublished TEM data reveal an inhomogeneous SnO layered structured thin films, related to shell-core structures. The harsh conclusion is that there is a combination of unrelated artefacts masquerading as ferroelectricity in powders and ALD films; and only a trace of a second phase in Cambridge PLD data suggests any perovskite content at all. The fact that X-ray, dielectric, and hysteresis data all lead to the wrong conclusion is instructive and reminds us of earlier work on copper calcium titanate (a well-known boundary-layer capacitor).
Based on density functional theory calculations and group theoretical analysis, we have studied NaLaMnWO$_{6}$ compound which has been recently synthesized [Phys. Rev. B 79, 224428 (2009)] and belongs to the $AABB{rm O}_{6}$ family of double perovskites. At low temperature, the structure has monoclinic $P2_{1}$ symmetry, with layered ordering of the Na and La ions and rocksalt ordering of Mn and W ions. The Mn atoms show an antiferromagnetic (AFM) collinear spin ordering, and the compound has been reported as a potential multiferroic. By comparing the low symmetry structure with a parent phase of $P4/nmm$ symmetry, two distortion modes are found dominant. They correspond to MnO$_{6}$ and WO$_{6}$ octahedron textit{tilt} modes, often found in many simple perovskites. While in the latter these common tilting instabilities yield non-polar phases, in NaLaMnWO$_{6}$ the additional presence of the $A$-$A^{}$ cation ordering is sufficient to make these rigid unit modes as a source of the ferroelectricity. Through a trilinear coupling with the two unstable tilting modes, a significant polar distortion is induced, although the system has no intrinsic polar instability. The calculated electric polarization resulting from this polar distortion is as large as $sim$ 16 ${mu}{rm C/cm^{2}}$. Despite its secondary character, this polarization is coupled with the dominant tilting modes and its switching is bound to produce the switching of one of two tilts, enhancing in this way a possible interaction with the magnetic ordering. The transformation of common non-polar purely steric instabilities into sources of ferroelectricity through a controlled modification of the parent structure, as done here by the cation ordering, is a phenomenon to be further explored.
We report on atomic ordering of B-site transition-metals and magnetic properties of epitaxial La2CrFeO6 double-perovskite films grown by pulsed-laser deposition under various conditions. The highest ordered sample exhibited a fraction of antisite-disorder of only 0.05 and a saturation magnetization of ~2mu_{B} per formula unit at 5 K. The result is consistent with the antiferromagnetic ordering of local spin moment (3d^{3}_{downarrow}3d^{5}_{uparrow}; S = -3/2+5/2 = 1). Therefore, the magnetic ground state of La2CrFeO6 double-perovskite that has been long debate is unambiguously revealed to be ferrimagnetic. Our results present a wide opportunity to explore novel magnetic properties of binary transition-metal perovskites upon epitaxial stabilization of the ordered phase.
We investigate a magnetic hysteresis loop with a remanent moment that couples to electric polarization to create coupled hysteretic multiferroic behavior in Lu2MnCoO6. Measurements of elastic neutron diffraction, muon spin relaxation, and micro-Hall magnetometry demonstrate an unusual mechanism for the magnetic hysteresis, namely the hysteretic evolution of a microscopic magnetic order, and not classic ferromagnetic domain effects. We show how the frustrated spin system evolves from antiferromagnetism with an incommensurate long-wavelength modulation and strong fluctuations towards a net magnetism. We also clarify the different temperature scales for the onset of ordering, dynamics, and hysteresis.
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