Cubic-tetragonal Transition Research Articles

Young’s modulus and damping of partially sintered potassium sodium niobate (KNN) ceramics and its temperature dependence determined via the impulse excitation technique (IET)

Potassium sodium niobate (KNN) is suitable for the preparation of lead-free piezoelectric ceramics, and thus the elastic properties and damping behavior should be reliably known for practical applications. In this work we compare partially sintered KNN ceramics prepared by uniaxial and conventional sintering from three different KNN powders and show that the impulse excitation technique (IET) is perfectly suited to determine Young’s modulus at room temperature as well as the temperature dependence of Young’s modulus and damping (heating-cooling cycles with maximum temperature 800 °C). Room temperature values of the effective Young’s modulus of partially sintered KNN ceramics are below the predictions for convex pores and above a recently published numerical benchmark relation for concave pores. The phase transition temperatures determined by the IET are 160–190 °C for the low-temperature orthorhombic-tetragonal / tetragonal-orthorhombic transition and 388–412 °C for the high-temperature tetragonal-cubic / cubic-tetragonal transition, with hysteresis 12–25 K.

Stabilizing Crystal Framework of an Overlithiated Li1+xMn2O4 Cathode by Heterointerfacial Epitaxial Strain for High-Performance Microbatteries.

To meet the increasing demands of high-energy and high-power-density lithium-ion microbatteries, overlithiated Li1+xMn2O4 (0 ≤ x ≤ 1) is an attractive cathode candidate due to the high theoretical capacity of 296 mAh g-1 and the interconnected lithium-ion diffusion pathways. However, overlithiation triggers the irreversible cubic-tetragonal phase transition due to Jahn-Teller distortion, causing rapid capacity degradation. In contrast to conventional lithium-ion batteries, microbatteries offer the opportunity to develop specific thin-film-based modification strategies. Here, heterointerfacial lattice strain is proposed to stabilize the spinel crystal framework of an overlithiated Li1+xMn2O4 (LMO) cathode by epitaxial thin film growth on an underlying SrRuO3 (SRO) electronic conductor layer. It is demonstrated that the lattice misfit at the LMO/SRO heterointerface results in an in-plane epitaxial constraint in the full LMO film. This suppresses the lattice expansion during overlithiation that typically occurs in the in-plane direction. It is proposed by density functional theory modeling that the epitaxial constraint can accommodate the internal lattice stress originating from the cubic-tetragonal transition during overlithiation. As a result, a doubling of the capacity is achieved by reversibly intercalating a second lithium ion in a LiMn2O4 epitaxial cathode with a complete reversible phase transition. An impressive cycling stability can be obtained with reversible capacity retentions of above 90.3 and 77.4% for the 4 and 3 V range, respectively. This provides an effective strategy toward a stable overlithiated Li1+xMn2O4 epitaxial cathode for high-performance microbatteries.

Polyhedral Distortions and Unusual Magnetic Order in Spinel FeMn2O4.

Spinel compounds AB2X4 consist of both tetrahedral (AX4) and octahedral (BX6) environments with the former forming a diamond lattice and the latter a geometrically frustrated pyrochlore lattice. Exploring the fascinating physical properties and their correlations with structural features is critical in understanding these materials. FeMn2O4 has been reported to exhibit one structural transition and two successive magnetic transitions. Here, we report the polyhedral distortions and their correlations to the structural and two magnetic transitions in FeMn2O4 by employing the high-resolution neutron powder diffraction. The cation distribution is found to be (Mn0.92+Fe0.13+)A(Mn3+Fe0.93+Mn0.12+)BO4. While large trigonal distortion is found even in the high-temperature cubic phase, the first-order cubic-tetragonal structural transition associated with the elongation of both tetrahedra and octahedra with shared oxygen atoms along the c axis occurs at TS ≈ 750 K, driven by the Jahn-Teller effect of the orbital active B-site Mn3+ cation. Strong magnetoelastic coupling is unveiled at TN1 ≈ 400 K as manifested by the appearance of Néel-type collinear ferrimagnetic order, an anomaly in both tetrahedral and octahedral distortions, as well as an anomalous decrease of the lattice constants c and a weak anomaly of a. Upon cooling to TN2 ≈ 65 K, it evolves to a noncollinear ferrimagnetic order accompanied by the different moments at the split magnetic sites B1 and B2. Only one-half of the B-site Mn3+/Fe3+ spins, i.e., the B2-site spins in the pyrochlore lattice, are canted, which is a unique magnetic order among spinels. The canting angle between A-site and B2-site moments is ∼25°, but the B1-site moment stays antiparallel to the A-site moment even at 10 K. This noncollinear order is accompanied by a modification of the O-B-O bond angles in the octahedra without significant change in lattice constants or tetrahedral/octahedral distortion parameters, indicating a distinct magnetoelastic coupling. We demonstrate distinct roles of the A-site and B-site magnetic cations in the structural and magnetic properties of FeMn2O4. Our study indicates that FeMn2O4 is a wonderful platform to unveil interesting magnetic order and to investigate their correlations with polyhedral distortions and lattice.

Effects of annealing temperature on structural phase transition and microstructure evolution of hydrothermally synthesized barium titanate nanoparticles

Commercial hydrothermally synthesized BaTiO3 powder with a cubic structure was annealed in a temperature range of 650 °C–900 °C, and the cubic-tetragonal structure transition and microstructure evolution of the powder were investigated in relation to the annealing process. The BaTiO3 powder used had a cubic structure below an annealing temperature of 800 °C and a tetragonal structure above 850 °C. Particle growth occurred under a low activation energy of ∼33.2 kJ mol−1 because of the nanocrystalline size effect, while the crystallite size slightly decreased in the powder with the cubic structure and sharply increased in that with the tetragonal structure. This was because the OH group in the powder with the cubic structure influenced the lattice extension on the particle surface. This stabilized the cubic structure and reduced the crystal ordering, which retarded the crystallite size. When the annealing temperature was increased, the crystallite growth reduced the intrinsic strain and enhanced the tetragonality in the powder with the tetragonal structure as a result of the removal of the OH group.

Unusual dynamics of the ferroelectric phase transition in K1−xLixTaO3 crystals

${\mathrm{K}}_{1\text{\ensuremath{-}}x}{\mathrm{Li}}_{x}\mathrm{Ta}{\mathrm{O}}_{3}$ ($x=0.043$, 0.08) crystals, characterized by pyroelectric current with calculated spontaneous polarization and zero-field second-harmonic generation, have been studied by broadband dielectric spectroscopy, including time-domain terahertz transmission and infrared (IR) reflectivity, and by polarized Raman spectroscopy in the 10--300 K temperature range. This multiexperimental approach has proven the percolative nature of the ferroelectric (FE) transition at low temperatures and demonstrated that the FE phase is inherently inhomogeneous and displays coexistence of FE and relaxor regions. Thanks to the very broad frequency range studied (from 1 Hz to 20 THz), the relevant excitations were identified and fitted in the dielectric response of both crystals: three relaxations, a central mode (CM), and a soft mode (SM) that splits into three components on cooling. Two Cole-Cole relaxations (assigned to flipping of polar nanoregions around the ${\mathrm{Li}}^{+}$ ions by $\ensuremath{\pi}/2$ and \ensuremath{\pi}, in agreement with the known literature), thermally activated below $\ensuremath{\sim}150\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, but staying in the gigahertz range at higher temperatures, do not show any frequency anomaly at the FE transition and are therefore related to the non-FE parts of the sample volume. A third thermally activated relaxation of unusually slow dynamics was revealed at low frequencies and preliminary assigned to an expected critical relaxation connected with the percolative nature of the FE phase transition. The IR SM, which undergoes much less softening than in the undoped ${\mathrm{KTaO}}_{3}$, splits into three components below the FE transition. Two higher-frequency components correspond to the FE volume part of the crystals assigned to the split ${A}_{1}$ and $E$ modes due to the cubic-tetragonal transition. The third low-frequency component is assigned to the non-FE (relaxor) volume part. Our assignment was confirmed by modeling the terahertz-IR response of the SM using the Bruggeman model within the effective medium approach. Below the SM response, an additional CM in the ${10}^{11}\phantom{\rule{0.16em}{0ex}}\mathrm{Hz}$ range in the whole temperature range is inferred from the fits.

Adsorption kinetics of methyl blue using metal-modified barium lanthanum titanate as an effective absorbent

Metal (Ag, Zn, and Na)-modified Ba–La–Ti-oxide (BLTO) was prepared, characterized, and applied as an adsorbent for methyl blue (MB) removal. Na–BaLa4Ti4O15 exhibited MB adsorption capacities of 37.6 mg g−1 while that for Zn–BaTiO3/BaLa2Ti3O10 was 33.0 mg g−1; both of these are eight times higher than that of commercial TiO2 powder. The MB adsorption proceeds according to Langmuir and Freundlich isotherms and fits the pseudo-2nd-order kinetic model, that the operative mechanism is chemisorption and involves strong adsorbate-adsorbent interactions. This could be attributed to the tuning of crystallinity by adding different metal ions to BLTO. For instance, Zn-doping induced the cubic-tetragonal transition, whereas Na-doping led to reduced crystallinity and a nonhomogeneous distribution of cations at B-sites of the BLTO structure. In summary, the BLTO, with appropriate metal modifications, shows promise as an absorbent for MB removal and future environmental remediation.

Optimizing K0.5Na0.5NbO3 Single Crystal by Engineering Piezoelectric Anisotropy.

K0.5Na0.5NbO3 is considered as one of the most promising lead-free piezoelectric ceramics in the field of wearable electronics because of its excellent piezoelectric properties and environmental friendliness. In this work, the temperature-dependent longitudinal piezoelectric coefficient was investigated in K0.5Na0.5NbO3 single crystals via the Landau–Ginzburg–Devonshire theory. Results show that the piezoelectric anisotropy varies with the temperature and the maximum of deviates from the polar direction of the ferroelectric phase. In the tetragonal phase, parallels with cubic polarization direction near the tetragonal-cubic transition region, and then gradually switches toward the nonpolar direction with decreasing temperatures. The maximum of in the orthorhombic phase reveals a distinct varying trend in different crystal planes. As for the rhombohedral phase, slight fluctuation of the maximum of was observed and delivered a more stable temperature-dependent maximum and its corresponding angle θmax in comparison with tetragonal and orthorhombic phases. This work not only sheds some light on the temperature-dependent phase transitions, but also paves the way for the optimization of piezoelectric properties in piezoelectric materials and devices.

Neutron-diffraction study of the cubic-tetragonal phase structural transition in the single crystals of the solid solutions of zirconium and yttrium oxides

Objectives. The determination of the phase relations, crystallographic characteristics, microstructure features, and atomic crystal structure of zirconium oxide crystals that are partially and completely stabilized by yttrium oxide additives, and the identification of the crystallographic and crystal-chemical correlations with the physicochemical properties of single crystals.Methods. The neutron structure of the crystals was studied using the neutron time-of-flight and constant wavelength methods using a high-resolution Fourier diffractometer on the IBR-2 pulsed fast reactor and a four-circle neutron diffractometer “Syntex.” Single crystals were grown by directed crystallization from the melts of mixtures (1 − х)ZrO2 ∙хY2O3 , х = 0.03 and х = 0.12 with different growth rates (10 and 40 mm/h).Results. It was observed that when growing single crystals with x = 0.03–0.05, the crystal was stratified into cubic and tetragonal phases, and the ratio between the phases depended on the growth rate. At a growth rate of 40 mm/h, the content of the cubic phase was insignificant. In the crystals of partially stabilized zirconium dioxide (ZrO2) with the additions of 3 mol % Y2O3, the coherent coexistence of cubic and tetragonal phases was established, and the twin law for a tetragonal component (rotation of unit cell axis by 90° around the a (b) axis) that was observed during the phase transition from high-temperature cubic phase to tetragonal phase was determined. For the fully stabilized zirconium oxide of the cubic symmetry (with 12 mol % Y2O3), the 0.3 Å displacements of oxygen atoms from their partial structural positions in the directions [100] and [111] were determined. These displacements correlated with the directions of the ion transport.Conclusions. Previous studies have shown that the ratio between the cubic and tetragonal phases of the single crystals of the ZrO2 –Y2O3 system depends on the growth rate of the single crystals. The content of Y2O3 in the cubic and tetragonal phases of a single crystal was determined using the non-destructive neutronography method on the same volume sample of a solid solution of this system. Moreover, the displacements of oxygen atoms from the main position of the crystal were determined.

Transport abnormity and its modulations via gating effect and light illumination at the SrNbO3/SrTiO3 interface

LaAlO3/SrTiO3-based two-dimensional electron gas (2DEG) has been extensively studied because of its intriguing physical properties and potential application prospect. However, seldom researches have related their extraordinary macroscopic transport phenomena to the microscopic domain structure of SrTiO3. This requires some unique technique like scanning superconducting quantum interference device (SQUID) microscopy. In this work, we developed a different 2DEG system at the interface of SrNbO3 thin film and SrTiO3. Using only the electrical methods, we found a pronounced hysteresis behavior in the resistance versus temperature curves, marked by the appearance/disappearance of two resistance peaks in the heating/cooling process. In sharp contrast to the conventional gate effect, the resistance peak grows under positive electric biases applied to backgate with conducting interface being grounded. In addition, a weak light (0.04 mW, 405 nm) can completely eliminate the two resistance anomalies. After a systematic analysis, we attribute the resistance anomaly to the cubic-tetragonal transition of bulk SrTiO3 and surface SrTiO3. The present work presents a promising demonstration to get mesoscopic information on oxide interface via transport behaviors.

A thermodynamic study on PbZr0.52Ti0.48O3 ceramic close to the tetragonal-cubic transition

The isobaric Gruneisen parameter and the wavenumber (frequency) of various Raman modes in PbZr1-xTixO3 (PZT x = 0.48) ceramic were calculated by means of the unit cell volume of this crystal. In addition, the damping constant (linewidth) of the Raman modes studied was computed from the pseudospin-phonon coupled and from the energy fluctuation models close to the tetragonal-cubic transition temperature of TC = 650 K. This calculation of the damping constant performed in terms of the order parameter (spontaneous polarization), which was associated with the wavenumbers of the Raman modes studied. Furthermore, the inverse relaxation time of the Raman modes in this ceramic calculated and the values of the activation energy were deduced in terms of the Arrhenius plot close to the tetragonal-cubic transition in PZT (x = 0.48) ceramic. Finally, the temperature dependence of some thermodynamic quantities, such as the isothermal compressibility and the specific heat of this ceramic, was predicted.

Structural and magnetic studies of Co1−xNixCr2O4 (0 ≤ x ≤ 1)

We have investigated the structural, magnetic, and thermal transport properties of Co1−xNixCr2O4 (0≤x≤1) to check the evolution of structural and magnetic properties upon the introduction of the Jahn–Teller active ion Ni2+. The x≤0.95 samples can be indexed with a cubic lattice with the space group Fd3m, whereas the x=0.98 sample undergoes a cubic-tetragonal structural transition, and the x>0.98 samples belong to a tetragonal lattice with the space group I41-amd at room temperature. All these samples undergo a paramagnetic–ferrimagnetic transition at TC. A spiral ferrimagnetic ordering at TS is suppressed when the amount of Ni is increased, and the abnormal magnetization at TS for the x=0.6 and 0.8 samples is not pronounced. The measurements of magnetic hysteresis reveal that the x=0.4 and 0.6 samples possess a rather large coercivity HC compared with the x=0 and 1 samples. Specifically, the x=0.4 and 0.6 samples undergo a short-range antiferromagnetic transition above TC. The magnetic properties of the Ni-doped samples can be understood in terms of the magnetic structure model consisting of longitudinal and transverse magnetic sublattices.

Anharmonic phonons and phase transitions in the vacancy-ordered double perovskite Cs2SnI6 from first-principles predictions

Tin-based vacancy-ordered double perovskite ${\mathrm{Cs}}_{2}{\mathrm{SnI}}_{6}$ has emerged as a potential candidate for nontoxic, nonhygroscopic, and high efficient light absorber in the field of photovoltaics. By using lattice-dynamics calculations, we demonstrate that a series of phase transitions in ${\mathrm{Cs}}_{2}{\mathrm{SnI}}_{6}$ from cubic ($Fm\overline{3}m$) to tetragonal ($P4/mnc$) and to monoclinic ($P{2}_{1}/n$) is driven by anharmonic phonon modes, associated with octahedral tilting. For all the anharmonic phonon soft modes, double-well potentials are found with different well depths, which are used to estimate the hopping rate of a structural transition between two symmetry-breaking local minima. Phonon calculations of the lower-symmetry structures reveal that the anharmonic phonon modes are partly or fully eliminated. Furthermore, we calculate Helmholtz free energy difference to predict the phase transition temperatures, which are 44 K for monoclinic-tetragonal and 137 K for tetragonal-cubic transitions.

Characterization of relaxor ferroelectrics from BiFeO3 doped (K0.5Na0.5)NbO3 systems

The additon of BiFeO3 (BFO) to (K0.5Na0.5)NbO3 (KNN) systems and termed as (l-x)KNN-xBFO for x=0 and x=0.075 were successfully synthesised using a combination of solid-state reaction and auto-combustion methods. The single phase of KNN-BFO was formed by two step of heat threatments; it is calcined at 575 °C for 6h then sintered at 1100 °C for 6h. The phase structure of KNN material changes from the orthorombic symmetry to tetragonal symmetry after the addition of BFO. The relaxor ferroelectric properties were carried out using impedance analyzer at various temperature and frequency. The pure KNN shows a typical ferroelecrics that has two Curie temperature at 220 °C (orthorombic-tetragonal transition) and 440 °C (tetragonal-cubic transition). After the 7.5%mol.BFO doped in the KNN system, the maxima peak around Tc becomes broader and its transition temperature becomes invisible then has a relaxor behaviours. The Currie-Weiss law is used to prove the ferroelectric of KNN (γ≤1) and relaxor of KNN-BFO (γ≥1).

Morphotropic phase boundary, field-induced transitions and dielectric properties of PbxSr1-xTiO3

A low-temperature rhombohedral-tetragonal morphotropic phase boundary (MPB) is discovered using a phenomenological approach in PbxSr1-xTiO3. Monoclinic structures are readily induced under electric field around the MPB which can be up-shifted to near room temperature by applying a negative pressure. Field-induced phase transition, polarization change, and dielectric response are computed. Under stress-free conditions monoclinic-orthorhombic transition can be induced in Pb0.5Sr0.5TiO3 by applying an electric field, giving rise to stronger field dependence of room-temperature dielectric constants as compared to Pb0.2Sr0.8TiO3. Under negative pressure, however, in Pb0.2Sr0.8TiO3 cubic-tetragonal transition can be induced, around which dielectric anomaly is sensitive to electric field, enabling significant enhancement of dielectric tunability.

Dielectric Response of Quantum Critical Ferroelectric as a Function of Pressure

In this work we report for the first time measurements of the dielectric loss of single-crystal SrTiO3 under the application of hydrostatic pressure up to 20 kbar and temperatures down to 200 mK which allow us to comment on the evolution of new fundamental material properties and their relationship with the recently discovered quantum critical phenomena in this material. The well known 18 K peak or shoulder was no longer observed after pressure was applied, even after subsequently removing it, suggesting it is associated with the twin walls formed at the 110 K cubic-tetragonal transition. The family of familiar peaks were all seen to increase in temperature linearly with pressure and the height of the 9.4 K peak was drastically suppressed by even the smallest pressures. This peak is discussed in the context of a postulated ferroelectric quantum critical point in SrTiO3 and the behaviour of its size linked to the position of this point on the recently established phase diagram.

Stability and Degradation in Hybrid Perovskites: Is the Glass Half-Empty or Half-Full?

Methylammonium lead iodide (CH3NH3PbI3) is an extensively used perovskite material with a remarkable potential for solar energy conversion. Despite its high photovoltaic efficiency, the material suffers from fast degradation when aging in atmospheric conditions and/or under sunlight. Here we review the principal degradation mechanisms of CH3NH3PbI3, focusing on the thermodynamic, environmental and polymorphic parameters that impact the stability of the material. A critical analysis of the available data indicates that degradation under ambient conditions is a defect-generation process that is highly localized on surfaces and interfaces, while it is further enhanced above the tetragonal-cubic transition at ∼54 °C. Within this context, we discuss the conservative role of N2 and propose strategies for the emergence of industrially viable hybrid photovoltaics.

Polymorphic phase transition and morphotropic phase boundary in Ba1−x Ca x Ti1−y Zr y O3 ceramics

This paper deals with Ca and Zr co-doped BaTiO3 (BCTZ(x, y)) (x = 0.1, 0.13, 0.2 and y = 0.05, 0.1, 0.15). These ceramics were prepared using the conventional solid state method. The symmetry, dielectric properties, Raman spectroscopy, ferroelectric behavior and piezoelectric effect were examined. X-ray diffraction (XRD) results display that morphotropic boundary occurs from tetragonal to orthorhombic region of BCZT(x=0.1, 0.2, y=0.05, 0.1) and polymorphic phase transitions from tetragonal to orthorhombic, orthorhombic to rhombohedral regions of BCZT(x=0.13, y=0.1). The evolution of the Raman spectra was investigated as a function of compositions at room temperature, in correlation with XRD analysis and dielectric measurements. We note that the substitution of Ca in Ba site and Zr ions in Ti site slightly decreased the cubic-tetragonal temperature transition (T C) and increased the orthorhombic–tetragonal (T 1) and rhombohedral–orthorhombic (T 2) temperatures transitions. The ferroelectric properties were examined by a P–E hysteresis loop. The two parameters ΔT 1 and ΔT 2 are defined as ΔT 1 = T C − T 1 and ΔT 2 = T C − T 2, they come close to T C for x = 0.13, y = 0.1, which reveals that this composition is around the polymorphic phase. The excellent piezoelectric coefficient of d 33 = 288 pC N−1, the electromechanical coupling factor k p = 40%, high constant dielectric 9105, coercive field E c = 0.32 (KV mm−1) and remanent polarization P r = 0.1 (µc mm−2) were obtained for composition x = 0.13, y = 0.1.

Effect of magnetic ion (Mn) doping on structural, ferroelectric and magnetic properties of Ba0.90Mn0.10TiO3

Synthesis, formation, structural phase transition, dielectric and magnetic properties of polycrystalline Ba0.90Mn0.10TiO3 (BMTO) ceramic have been investigated to study the effect of magnetic ion (Mn) doping at Ba-site in the well-known ferroelectric (FE) material BaTiO3 (BTO). The polycrystalline ceramic BMTO has been prepared via solid state reaction route. The Wyckoff-site occupancy of the substituted Mn has been determined through x-ray diffraction (XRD) and soft X-ray absorption measurement via confirmation of its oxidation state to be 2+ and that of Ti to be 4+. The temperature dependent XRD, dielectric and pyroelectric studies show all the known transitions similar to the pristine BTO. However, the transition temperatures (Tc) were found shifted towards lower temperatures as compare to the pristine BTO. The shifts in Tc corresponding to cubic-tetragonal and tetragonal-orthorhombic transitions are significant as compared to that of orthorhombic-rhombohedral transition. Frequency dependent dielectric measurements show the space-charge related dielectric relaxation. The dc-magnetization measurements show antiferromagnetic interactions in BMTO. Our results also address the role of Ba site on the ferroelectric properties of BTO.

Calculation of the raman frequency, damping constant (Linewidth) and the relaxation time near the tetragonal-cubic transition in PbTiO3

Frequencies, damping constants and the relaxation times of some Raman modes including the two soft modes are calculated as a function of pressure near the tetragonal-cubic transition in PbTiO3. Calculation of the Raman frequencies is performed using the observed volume data from the literature by means of the mode Grüneisen parameter at various pressures. Pressure dependence of the damping constant and the relaxation time is predicted using the pseudospin-phonon coupled model and the energy fluctuation model by considering that the Raman frequency can be taken as the order parameter (spontaneous polarization) for the tetragonal-cubic transition in PbTiO3. Expressions from both models for the damping constants are fitted to the observed Raman linewidths of the two soft modes and for the other Raman modes the damping constant and the relaxation time are predicted close to the transition. We find that damping constants diverge and the inverse relaxation time decreases for the soft modes with increasing pressure near the critical pressure (PC∼ 11GPa). The other Raman modes exhibit unusual critical behavior.Our results indicate that the observed behavior of the Raman frequencies can be predicted from the volume data through the mode Grüneisen parameter for the tetragonal-cubic transition in PbTiO3. The damping constant and the relaxation time for the Raman modes can also be predicted adequately using the pseudospin-phonon coupled model and the energy fluctuation model to explain the mechanism of the phase transition between the tetragonal and cubic phases in PbTiO3.

First determination of the valence band dispersion of CH3NH3PbI3 hybrid organic–inorganic perovskite

The family of hybrid organic–inorganic halide perovskites is in the limelight because of their recently discovered high photovoltaic efficiency. These materials combine photovoltaic energy conversion efficiencies exceeding 22% and low-temperature and low-cost processing in solution; a breakthrough in the panorama of renewable energy. Solar cell operation relies on the excitation of the valence band electrons to the conduction band by solar photons. One factor strongly impacting the absorption efficiency is the band dispersion. The band dispersion has been extensively studied theoretically, but no experimental information was available. Herein, we present the first experimental determination of the valence band dispersion of methylammonium lead halide in the tetragonal phase. Our results pave the way for contrasting the electronic hopping or the electron effective masses in different theories by comparing to our experimental bands. We also show a significant broadening of the electronic states, promoting relaxed conditions for photon absorption, and demonstrate that the tetragonal structure associated to the octahedra network distortion below 50 °C induces only a minor modification of the electronic bands, with respect to the cubic phase at high temperature, thus minimizing the impact of the cubic-tetragonal transition on solar cell efficiencies.