Room Temperature Tetragonal Phase Research Articles

Bandgap Pressure Coefficient of a CH3NH3PbI3 Thin Film Perovskite.

Recent scientific interest in examining the bandgap evolution of a MAPbI3 hybrid perovskite by applying hydrostatic pressure has mostly focused on a room-temperature tetragonal phase. In contrast, the pressure response of a low-temperature orthorhombic phase (OP) of MAPbI3 has not been explored and understood. In this research, we investigate for the first time how hydrostatic pressure alters the electronic landscape of the OP of MAPbI3. Pressure studies using photoluminescence combined with calculations within density functional theory at zero temperature allowed us to identify the main physical factors affecting the bandgap evolution of the OP of MAPbI3. The negative bandgap pressure coefficient was found to be strongly dependent on the temperature (α120K = -13.3 ± 0.1 meV/GPa, α80K = -29.8 ± 0.1 meV/GPa, and α40K = -36.3 ± 0.1 meV/GPa). Such dependence is related to the changes in the Pb-I bond length and geometry in the unit cell as the atomic configuration approaches the phase transition as well as the increasing phonon contribution to octahedral tilting as the temperature increases.

Anharmonic organic cation vibrations in the hybrid lead halide perovskite CH3NH3PbI3

Ultrafast vibrational dynamics of inorganic-organic hybrid lead halide perovskite CH3NH3PbI$ in the room-temperature tetragonal phase is investigated under moderate photoexcitation slightly below the band gap. Time-resolved optical Kerr effect signal exhibits periodic modulations due to the libration and twisting of the methylammonium molecule at 4 and 8 THz, in addition to the inorganic sublattice deformation at 1.2 THz. These oscillations are heavily damped, and their frequencies exhibit blueshifts with increasing pump density, contrary to the expectation from conventional laser heating. Our ab initio lattice dynamics simulations reveal strongly anharmonic vibrational potentials for the zone-center optical phonon modes, and thereby confirm the anharmonicity to be the origin of the experimentally observed frequency blueshifts.

Elucidating the atomistic origin of anharmonicity in tetragonal CH3NH3PbI3 with Raman scattering

Halide perovskite (HP) semiconductors exhibit unique strong coupling between the electronic and structural dynamics. The high-temperature cubic phase of HPs is known to be entropically stabilized, with imaginary frequencies in the calculated phonon dispersion relation. Similar calculations, based on the static average crystal structure, predict a stable tetragonal phase with no imaginary modes. This work shows that in contrast to standard theory predictions, the room-temperature tetragonal phase of CH$_{3} $NH$_{3} $PbI$_{3}$ is strongly anharmonic. We use Raman polarization-orientation (PO) measurements and \textit{ab initio} molecular dynamics (AIMD) to investigate the origin and temperature evolution of the strong structural anharmonicity throughout the tetragonal phase. Raman PO measurements reveal a new spectral feature that resembles a soft mode. This mode shows an unusual continuous increase in damping with temperature which is indicative of an anharmonic potential surface. The analysis of AIMD trajectories identifies two major sources of anharmonicity: the orientational unlocking of the [CH$_{3} $NH$_{3}$]$^+$ ions and large amplitude octahedral tilting that continuously increases with temperature. Our work suggests that the standard phonon picture cannot describe the structural dynamics of tetragonal CH$_{3} $NH$_{3} $PbI$_{3}$.

Looking beyond the Surface: The Band Gap of Bulk Methylammonium Lead Iodide.

Despite the intense research on photovoltaic lead halide perovskites, reported optical properties as basic as the absorption onset and the optical band gap vary significantly. To unambiguously answer the question whether the discrepancies are a result of differences between bulk and "near-surface" material, we perform two nonlinear spectroscopies with drastically different information depths on single crystals of the prototypical (CH3NH3)PbI3 methylammonium lead iodide. Two-photon absorption, detected via the resulting generation of carriers and photocurrents (2PI-PC), probes the interband transitions with an information depth in the millimeter range relevant for bulk (single-crystal) material. In contrast, the transient magneto-optical Kerr effect (trMOKE) measured in a reflection geometry determines the excitonic transition energies in the region near (hundreds of nm) the surface which also determine the optical properties in typical thin films. To identify differences between structural phases, we sweep the sample temperature across the orthorhombic-tetragonal phase transition temperature. In the application-relevant room-temperature tetragonal phase (at 170 K), we find a bulk band gap of 1.55 ± 0.01 eV, whereas in the near-surface region excitonic transitions occur at 1.59 ± 0.01 eV. The latter value is consistent with previous reflectance measurements by other groups and considerably higher than the bulk band gap. The small band gap of the bulk material explains the extended infrared absorption of crystalline perovskite solar cells, the low-energy bands which carry optically driven spin-polarized currents, and the narrow bandwidth of crystalline perovskite photodetectors making use of the spectral filtering at the surface.

Theoretical investigation of surface electronic structure and thermodynamic energies of (1x1) polar and nonpolar K1/2Bi1/2TiO3 (001) surfaces

Theoretical investigations are carried to explore surface electronic structure and surface energetics of (1 × 1) polar and nonpolar (001) surfaces of room temperature tetragonal phase of lead-free relaxor ferroelectric K1/2Bi1/2TiO3(KBT) within the framework of density functional theory. In particular, polar (KO)-, (BiO)+ and non-polar (TiO2)0 terminations with bulk P4mm symmetry and non-polar (K1/2Bi1/2O)0 and (TiO2)0 terminations with bulk P4bm symmetry are explored. The electronic structures of different terminations differ significantly with respect to bulk and with each other due to surface charge compensation. In case of BiO termination, the Fermi level shifts just above the conduction band minimum (CVM), whereas in case of KO termination, it shifts just below the valence band maximum (VBM) resulting in formation of localized surface states in the band gap with conduction and valence band character. In addition, localized surface states are also formed in the gap at ~11 eV below the VBM. These deep energy localized surface states may be expected to have important implications for surface relaxation and thermodynamic stability of surfaces and their reconstructions. For polar surfaces, surface relaxations are found to be strongly dependent on surface charge and are quite disparate as compared to that for non-polar surfaces. Surface energy of K1/2Bi1/2O termination is found to be much smaller than KO, BiO and TiO2 terminations, suggesting that stacking sequence -K1/2Bi1/2O–TiO2–K1/2Bi1/2O–TiO2- of non-polar planes is significantly favorable energetically than the sequence -KO-TiO2-BiO-TiO2- KO- of polar and non-polar planes in KBT. The preferred stacking sequence of non-polar planes may be expected to have significant influence on the degree of A-site cation ordering in K1/2Bi1/2TiO3.

Single-crystal x-ray diffraction and impedance spectroscopy investigations of the RbxCs1-xH2PO4 (0≤x≤1) proton conductor series

We have used single-crystal x-ray diffraction to investigate the structural modifications induced by Rb-doping of the superprotonic conductor CsH2PO4. We found that the monoclinic P21/m CsH2PO4 modification persists within the RbxCs1-xH2PO4 (0 ≤ x ≤ 1) series upon Rb-doping from x = 0.1 to x = 0.7. Rb0.8Cs0.2H2PO4 (x = 0.8), however, exhibits a previously unreported P21/c monoclinic phase, where the mirror plane is lost and disorder is present in the PO4 tetrahedra even at room temperature. Higher levels of x display a tetragonal I-42d unit cell isomorphic with the known structure of RbH2PO4. The temperature dependence of the proton conductivity determined from impedance spectroscopy data collected within the 160⁰C-250 °C range is also markedly different at high Rb-doping levels, x ≥ 0.8. Finally, we found that Rb0.9Cs0.1H2PO4 undergoes a transition from its room-temperature tetragonal I-42d phase to an intermediate-temperature monoclinic P21/m modification at a significantly lower temperature (∼80 °C) than its RbH2PO4 counterpart (∼120 °C).

Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH3NH3PbI3**The work at Fudan University was supported by the Special Funds for Major State Basic Research, National Natural Science Foundation of China (NSFC), and Project of Shanghai Municipality (16520721600). S.C. was supported by NSFC under Grant No 91233121, Shanghai Rising-Star Program (14QA1401500) and CC of ECNU. The work at Bath was supported

The organic-inorganic hybrid perovskite CH3NH3PbI3 has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of CH3NH3PbI3 solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperature tetragonal phase of CH3NH3PbI3 is thermodynamically unstable with respect to the phase separation into CH3NH3I + PbI2, i.e., the disproportionation is exothermic, independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombic phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH3NH3PbI3 is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH3NH3 by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH3NH3PbI3 and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells.

Assessing Density Functionals Using Many Body Theory for Hybrid Perovskites.

Which density functional is the "best" for structure simulations of a particular material? A concise, first principles, approach to answer this question is presented. The random phase approximation (RPA)-an accurate many body theory-is used to evaluate various density functionals. To demonstrate and verify the method, we apply it to the hybrid perovskite MAPbI_{3}, a promising new solar cell material. The evaluation is done by first creating finite temperature ensembles for small supercells using RPA molecular dynamics, and then evaluating the variance between the RPA and various approximate density functionals for these ensembles. We find that, contrary to recent suggestions, van der Waals functionals do not improve the description of the material, whereas hybrid functionals and the strongly constrained appropriately normed (SCAN) density functional yield very good agreement with the RPA. Finally, our study shows that in the room temperature tetragonal phase of MAPbI_{3}, the molecules are preferentially parallel to the shorter lattice vectors but reorientation on ps time scales is still possible.

Unraveling the Exciton Binding Energy and the Dielectric Constant in Single-Crystal Methylammonium Lead Triiodide Perovskite.

We have accurately determined the exciton binding energy and reduced mass of single crystals of methylammonium lead triiodide using magneto-reflectivity at very high magnetic fields. The single crystal has excellent optical properties with a narrow line width of ∼3 meV for the excitonic transitions and a 2s transition that is clearly visible even at zero magnetic field. The exciton binding energy of 16 ± 2 meV in the low-temperature orthorhombic phase is almost identical to the value found in polycrystalline samples, crucially ruling out any possibility that the exciton binding energy depends on the grain size. In the room-temperature tetragonal phase, an upper limit for the exciton binding energy of 12 ± 4 meV is estimated from the evolution of 1s-2s splitting at high magnetic field.

Charge carrier mobility in hybrid halide perovskites

The charge transport properties of hybrid halide perovskites are investigated with a combination of density functional theory including van der Waals interaction and the Boltzmann theory for diffusive transport in the relaxation time approximation. We find the mobility of electrons to be in the range 5–10 cm2V−1s−1 and that for holes within 1–5 cm2V−1s−1, where the variations depend on the crystal structure investigated and the level of doping. Such results, in good agreement with recent experiments, set the relaxation time to about 1 ps, which is the time-scale for the molecular rotation at room temperature. For the room temperature tetragonal phase we explore two possible orientations of the organic cations and find that the mobility has a significant asymmetry depending on the direction of the current with respect to the molecular axis. This is due mostly to the way the PbI3 octahedral symmetry is broken. Interestingly we find that substituting I with Cl has minor effects on the mobilities. Our analysis suggests that the carrier mobility is probably not a key factor in determining the high solar-harvesting efficiency of this class of materials.

High pressure synchrotron x-ray diffraction studies of superprotonic transitions in phosphate solid acids

We have used high-pressure synchrotron x-ray diffraction to investigate the structural modifications that accompany the abrupt, several-order-of-magnitude proton conductivity increase exhibited upon heating by CsH2PO4 (CDP) and RbH2PO4 (RDP). Our temperature-resolved data collected on polycrystalline CDP subjected to a pressure P=1GPa reveal a polymorphic structural transition at T~260°C from this solid acid's room-temperature monoclinic (P21/m) phase to a stable cubic (Pm3m, a=4.88Å) modification. The transition occurs under the same (P, T) conditions as the 1000-fold jump in CDP's proton conductivity, and Rietveld analysis confirms that the high-pressure phase has essentially the same crystal structure as the cubic CDP polymorph previously observed under a saturated water vapor atmosphere. For RDP we find a structural transformation from its room temperature tetragonal (I-42d) phase to an intermediate temperature monoclinic (P21/m) modification, which, remarkably, is isomorphic to its monoclinic CDP counterpart. This suggests that a monoclinic→cubic structural transition, similar to the one observed in CDP, is responsible for RDP's superprotonic behavior. We present the first direct evidence for the existence of a high temperature cubic RDP phase.

Temperature-induced isostructural phase transition, associated large negative volume expansion, and the existence of a critical point in the phase diagram of the multiferroic(1--x)BiFeO3–xPbTiO3solid solution system

We report here an unusual ferroelectric to ferroelectric isostructural phase transition and associated giant negative thermal expansion (NTE) for the tetragonal composition x = 0.31 closest to the morphotropic phase boundary (MPB) of the multiferroic (1--x)BiFeO${}_{3}$--xPbTiO${}_{3}$ (BF-xPT) solid solution system. It is shown that the room temperature tetragonal phase (T${}_{1}$) of BF-0.31PT with extremely large tetragonality undergoes a first-order isostructural phase transition to another tetragonal phase (T${}_{2}$) with lower tetragonality without losing the P4$mm$ space group symmetry and the occupied Wyckoff positions. The T${}_{2}$ phase finally transforms into the paraelectric cubic phase at still higher temperatures. Using group theoretical considerations, we show that the observed atomic displacements associated with this isostructural phase transition correspond to specific irreducible representations of the P4$mm$ space group at its Brillouin zone center, and as such this transition may be phonon driven. Pronounced anomalies in the thermal displacement parameters at the T${}_{1}$ to T${}_{2}$ transition provide evidence for such a phonon-mediated isostructural phase transition. The high tetragonality ferroelectric phase (T${}_{1}$) of BF-0.31PT shows the largest NTE coefficient reported so far in the mixed BF-xPT system. The isostructural transition is shown to persist for tetragonal compositions of BF-xPT up to x = 0.60. A complete phase diagram of the BF-xPT system showing the existence of a critical point at T \ensuremath{\sim} 677 K for x \ensuremath{\approx} 0.63 is also presented.

Low Temperature Phase Diagram of NH3BH3

ABSTRACTThe pressure-temperature (p-T) phase diagram of NH3BH3 has been investigated by thermal conductivity measurements up to 1.5 GPa at temperatures between 100 and 300 K, and the phase boundaries between the three known structural phases have been identified. The transformation between the room temperature tetragonal I4mm phase and the low temperature orthorhombic Pmn21 phase (Tc = 218 K at p = 0) shows only a small hysteresis. The transformation into the high pressure orthorhombic Cmc21 phase (at 1.0 GPa near 292 K) has a very strong hysteresis, up to Δp = 0.5 GPa, and below 230 K a fraction of this phase is metastable even at atmospheric pressure.

Transformation stress induced metastable tetragonal phase in (93–92)%Pb(Zn1/3Nb2/3)O3–(7–8)%PbTiO3 single crystals

High-resolution synchrotron radiation diffraction results indicate that the room temperature tetragonal (T) phase in (93–92)%Pb(Zn1/3Nb2/3)O3–(7–8)%PbTiO3 [PZN–(7–8)%PT] single crystals is a metastable phase stabilized by the transformation-cum-cooling stress in the crystal. This is manifested by the “missing” (001)T diffractions when the diffractions were taken from (001) fractured surfaces. The stress relaxation in this case has caused the metastable (001)T domains in the surface layer to transform to the equilibrium rhombohedral phase. Temperature and particle size effects on the metastable domains have been examined and discussed. Based on the present finding, a new PZN–PT phase diagram showing extended rhombohedral+tetragonal region is proposed.

Intermediate-temperature polymorphic phase transition in KH 2PO 4: A synchrotron X-ray diffraction study

We have used synchrotron X-ray diffraction to investigate the structural and chemical changes undergone by polycrystalline KH 2PO 4 (KDP) upon heating within the 30–250 °C temperature interval. Our data show evidence of a polymorphic transition at T∼190 °C from the room-temperature tetragonal KDP phase to a new intermediate-temperature monoclinic KDP modification (spacegroup P2 1/ m and lattice parameters a=7.590, b=6.209, c=4.530 Å, and β=107.36°). The monoclinic RDP polymorph remains stable upon further heating to 235 °C, and is isomorphic to its RbH 2PO 4 and CsH 2PO 4 counterparts.

First-principles study on the lattice dynamics and thermodynamics properties of CaFe 2As 2

The lattice dynamics of room temperature tetragonal phase of CaFe 2As 2 is studied by first-principles calculations based on the ab initio pseudopotential and plane wave basis. Phonon spectrum throughout the Brillouin zone is obtained using the density-functional perturbation theory within the linear response method. The frequencies of the Raman active modes, phonon density of states and lattice heat capacity are compared with the experimental measurements. The differences between our calculations and experimental measurements in the high-frequency region in phonon density of states are due to the interplay between phonons and spin.

Evidence from neutron diffraction for superconductivity in the stabilized tetragonal phase ofCaFe2As2under uniaxial pressure

${\text{CaFe}}_{2}{\text{As}}_{2}$ single crystals under uniaxial pressure applied along the $c$ axis exhibit the coexistence of several structural phases at low temperatures. We show that the room-temperature tetragonal phase is stabilized at low temperatures for pressures above 0.06 GPa, and its weight fraction attains a maximum in the region where superconductivity is observed under applied uniaxial pressure. Simultaneous resistivity measurements strongly suggest that this phase is responsible for the superconductivity in ${\text{CaFe}}_{2}{\text{As}}_{2}$ found below 10 K in samples subjected to nonhydrostatic pressure conditions.

Effect of Oxygen Vacancy on Phase Transition and Photoluminescence Properties of Nanocrystalline Zirconia Synthesized by the One-Pot Reaction

Nanocrystalline ZrO2 fine powders were prepared via the one-pot reaction followed by annealing from 700 to 1100 °C in air. It is believed that generation of excess oxygen vacancies within nanocrystalline ZrO2 is primarily responsible for room-temperature tetragonal phase stabilization below a critical size, and the phase transformation from tetragonal to monoclinic ZrO2 happened in the annealing process. Luminescent properties of ZrO2 were greatly affected by oxygen vacancies in the phase transition process. Two bands centered at 350 and 470 nm were observed in tetragonal ZrO2. The electrons trapped by oxygen vacancies creating F centers recombined with holes yielding the 350 nm luminescence. Temporal decay of the 470 nm luminescence is due to the detrapping of electrons to the Ti3+ luminescence centers. Elevated temperature accelerated the phase transformation to monoclinic ZrO2 and decreased the oxygen vacancy concentration, resulting in a decrease of the 350 nm emission and enhancement of the 470 nm ph...

Temperature evolution of lattice strains in relaxor PbSc1∕2Nb1∕2O3 thin films

Temperature evolution of the lattice parameters of relaxor PbSc1∕2Nb1∕2O3 (PSN) thin films have been measured and compared to the evolution of bulk PSN. In PSN films, a room temperature tetragonal polar phase transforms at higher temperature into a nonpolar tetragonal phase. Critical and Burns temperatures, associated, respectively, to the onset of the ferroelectric phase and to the nucleation of polar nanoregions have been assigned from strong anomalies in the temperature evolution of the film. These complex behaviors are explained by a quantitative study of the strains lying in the film.

Low-temperature structural phase transition and incommensurate lattice modulation in the spin-gap compoundBaCuSi2O6

Results of high resolution x-ray diffraction experiments are presented for single crystals of the spin gap compound BaCuSi$_2$O$_6$ in the temperature range from 16 to 300 K. The data show clear evidence of a transition from the room temperature tetragonal phase into an incommensurately modulated orthorhombic structure below $\sim$100 K. This lattice modulation is characterized by a resolution limited wave vector {\bf q}$_{IC}$=(0,$\sim$0.13,0) and its 2$^{nd}$ and 3$^{rd}$ harmonics. The phase transition is first order and exhibits considerable hysteresis. This observation implies that the spin Hamiltonian representing the system is more complex than originally thought.