Polycrystalline Form Research Articles

Plastic/ferroelectric molecular crystals: Ferroelectric performance in bulk polycrystalline forms

Plastic crystals are a class of compounds composed of small molecules with globular structures. Plastic crystals exhibit various unique features, most of which arise from the rotator motion and orientational disorder of the constituent molecules. We recently discovered that some plastic crystals exhibit ferroelectricity in lower-temperature phases, including the room-temperature phase. The crystals, i.e., plastic/ferroelectric molecular crystals, have demonstrated several unique features that differ from conventional molecular ferroelectrics. In particular, the materials have achieved ferroelectric performance for the first time as bulk polycrystals of small-molecule crystals. These features are attributed to the multiaxial ferroelectricity and facile preparation of monolithic bulk polycrystals, which are impossible to achieve for conventional molecular crystals. Numerous related molecular ferroelectrics have been developed, rendering plastic/ferroelectric crystals an emerging class of molecular ferroelectrics. In this perspective, we have outlined the development and unique features of plastic/ferroelectric molecular crystals, focusing particularly on their ferroelectricity and related properties in bulk polycrystalline forms.

Anomalous electronic properties in layered, disordered ZnVSb

New materials discovery is the driving force for progress in solid state physics and chemistry. Here we solve the crystal structure and comprehensively study physical properties of ZnVSb in the polycrystalline form. Synchrotron x-ray diffraction reveals that the compound attains a layered ZrSiS-type structure ($P4$/nmm, $a$ = 4.09021(2) \AA{}, $c$ = 6.42027(4) \AA{}). The unit cell is composed of a 2D vanadium network separated by Zn-Sb blocks that are slightly distorted from the ideal cubic arrangement. A considerable amount of vacancies were observed on the vanadium and antimony positions; the experimental composition is ${\mathrm{ZnV}}_{0.91}{\mathrm{Sb}}_{0.96}$. Low-temperature x-ray diffraction shows very subtle discontinuity in the lattice parameters around 175 K. Bonding V-V distance is below the critical separation of 2.97 \AA{} known from the literature, which allows for V-V orbital overlap and subsequent metallic conductivity. From ab initio calculations, we found that ZnVSb is a pseudogap material with an expected dominant vanadium contribution to the density of states at the Fermi level. The energy difference between the antiferromagnetic and nonordered magnetic configurations is rather small (0.34 eV/f.u.). X-ray photoelectron spectroscopy fully corroborates the results of the band structure calculations. Magnetic susceptibility uncovered that, in ZnVSb, itinerant charge carriers coexist with a small, localized magnetic moment of ca. 0.25 ${\ensuremath{\mu}}_{\mathrm{B}}$. The itinerant electrons arise from the ordered part of the vanadium lattice. Fractional localization, in turn, was attributed to V atoms neighboring vacancies, which hinder full V-V orbital overlap. Furthermore, the susceptibility and electrical resistivity showed a large hysteresis between 120 K and 160 K. The effect is not sensitive to magnetic fields up to 9 T. Curie-Weiss fitting revealed that the amount of itinerant charge carriers in ZnVSb drops with decreasing temperature below 160 K, which is accompanied by a concurrent rise in the localized magnetic moment. This observation together with the overall shape of the hysteresis in the resistivity allows for suggesting a plausible origin of the effect as a charge-transfer metal-insulator transition. Ab initio phonon calculations show the formation of a collective phonon mode at 2.8 THz (134 K). The experimental heat capacity reflected this feature by a boson peak with Einstein temperature of 116 K. Analysis of the heat capacity with both an ab initio perspective and Debye-Einstein model revealed a sizable anharmonic contribution to heat capacity, in line with disordered nature of the material. Further investigation of the electron and phonon properties for ZnVSb is likely to provide valuable insight into the relation between structural disorder and the physical properties of transition-metal-bearing compounds.

Structure and properties of nano- and polycrystalline Mn-doped CuCr2Se4 obtained by ceramic method and high-energy ball milling

Chemical compounds with a general formula CuCr2–xMnxSe4 (x = 0.1, 0.2) were successfully synthesized in polycrystalline form, using solid state reaction method, and as nanoparticles, using high-energy ball milling. The influence of manganese and grain size on their structure, magnetic and thermal properties was investigated by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), magnetic measurements (SQUID, high magnetic fields) and DSC (Differential Scanning Calorimetry). Electronic structures of obtained compounds were determined using XANES (X-ray Near Edge Absorption Spectroscopy) and XPS (X-ray Photoelectron Spectroscopy). X-ray studies confirmed that the spinel phase forms only for x = 0.1, 0.2. The obtained compounds crystallize with the space group Fd3¯m. The results of magnetic measurements for poly- and nanocrystalline compounds showed that the polycrystalline compounds are ferromagnets with the Curie temperature of TC =352 K for x = 0.1 and TC =342 K for x = 0.2, and the Curie-Weiss temperature of θCW =362 K for x = 0.1 and θCW =354 K for x = 0.2. Whereas the nanoparticles CuCr2–xMnxSe4 (x = 0.1, 0.2) revealed a superparamagnetic character with blocking temperature of TB ≈ 25 K for both compounds.The XANES and XPS data indicate the Cr ions are trivalent (Cr3+). Mn ions are trivalent and tetravalent (Mn3+ and Mn4+), which implies small amount of the Cu ions change the valency from divalency to monovalency (Cu2+ and Cu1+). DSC/TG analysis indicates that the presence of Mn ions in the CuCr2Se4 crystal lattice affects the stability and resistance of the doped compounds in comparison with the pure CuCr2Se4. Decreasing the grain size leads to a decrease in melting temperature.

Impact of Aliovalent Alkaline-Earth metal solutes on Ceria Grain Boundaries: A density functional theory study

Ceria has proven to be an excellent ion-transport and ion-exchange material when used in polycrystalline form and with a high-concentration of aliovalent doped cations. Despite its widespread application, the impact of atomic-scale defects in this material are scarcely studied and poorly understood. In this article, using first-principles simulations, we provide a fundamental understanding of the atomic-structure, thermodynamic and electronic properties of undoped grain-boundaries (GBs) and alkaline-earth metal (AEM) doped GBs in ceria. Using density-functional theory simulations, with a GGA+U functional, we find the Σ3 (111)/[1¯01] GB is energetically more stable than the Σ3 (121)/[1¯01] GB due to the larger atomic coherency in the Σ3 (111)/[1¯01] GB plane. We dope the GBs with ∼20% [M]GB (M=Be, Mg, Ca, Sr, and Ba) and find that the GB energies have a parabolic dependence on the size of solutes, the interfacial strain and the packing density of the GB. We see a stabilization of the GBs upon Ca, Sr and Ba doping whereas Be and Mg render them energetically unstable. The electronic density of states reveal that no defect states are present in or above the band gap of the AEM doped ceria, which is highly conducive to maintain low electronic mobility in this ionic conductor. The electronic properties, unlike the energetic properties, exhibit complex inter-dependence on the structure and chemistry of the host and the solutes. This work makes advances in the atomic-scale understanding of aliovalent cation doped ceria GBs serving as an anchor to future studies that can focus on understanding and improving ionic-transport.

Bimetallic Trifluoroacetates as Precursors to Layered Perovskites A2MnF4 (A = K, Rb, and Cs).

Four novel alkali-manganese(II) trifluoroacetates were synthesized, and their potential as self-fluorinating precursors to layered perovskites A2MnF4 (A = K, Rb, and Cs) was demonstrated. Na2Mn(tfa)4, K4Mn2(tfa)8, Rb4Mn2(tfa)8·0.23H2O, and Cs3Mn2(tfa)7(tfaH) (tfa = CF3COO- and tfaH = CF3COOH) were grown as single crystals, and their crystal structures solved using X-ray diffraction. Chemically pure K4Mn2(tfa)8, Rb4Mn2(tfa)8·0.23H2O, and Cs3Mn2(tfa)7(tfaH) were also prepared in polycrystalline form as confirmed by thermal analysis and powder X-ray diffraction. Thermolysis of these powders yielded the isostructural series K2MnF4, Rb2MnF4, and Cs2MnF4 at low temperatures (≈200-300 °C). Trifluoromethyl groups belonging to the trifluoroacetato ligands served as the fluorine source, thereby eliminating the need for external fluorinating agents. K2MnF4 and Rb2MnF4 were obtained as single-phase powders, whereas Cs2MnF4 crystallized along with CsMnF3. Access to polycrystalline Cs2MnF4 coupled to Rietveld analysis enabled elucidation of the crystal structure of this ternary fluoride, which had remained elusive. Findings presented in this article expand the synthetic accessibility of polycrystalline A2MnF4 fluorides, for which a scarce number of routes is available in the literature.

Microfluidic Approach for Lead Halide Perovskite Flexible Phototransistors

Lead halide perovskites possess outstanding optical characteristics that can be employed in the fabrication of phototransistors. However, due to low current modulation at room temperature, sensitivity to the ambient environment, lack of patterning techniques and low carrier mobility of polycrystalline form, investigation in perovskite phototransistors has been limited to rigid substrates such as silicon and glass to improve the film quality. Here, we report on room temperature current modulation in a methylammonium lead iodide perovskite (MAPbI3) flexible transistor made by an extremely cheap and facile fabrication process. The proposed phototransistor has the top-gate configuration with a lateral drain–channel–source structure. The device performed in the linear and saturation regions both in the dark and under white light in different current ranges according to the illumination conditions. The transistor showed p-type transport characteristics and the field effect mobility of the device was calculated to be ~1.7 cm2 V−1 s−1. This study is expected to contribute to the development of MAPbI3 flexible phototransistors.

Scalable Fabrication of Molybdenum Disulfide Nanostructures and their Assembly.

Molybdenum disulfide (MoS2 ) is a multifunctional material that can be used for various applications. In the single-crystalline form, MoS2 shows superior electronic properties. It is also an exceptionally useful nanomaterial in its polycrystalline form with applications in catalysis, energy storage, water treatment, and gas sensing. Here, the scalable fabrication of longitudinal MoS2 nanostructures, i.e., nanoribbons, and their oxide hybrids with tunable dimensions in a rational and well-reproducible fashion, is reported. The nanoribbons, obtained at different reaction stages, that is, MoO3 , MoS2 /MoO2 hybrid, and MoS2 , are fully characterized. The growth method presented herein has a high yield and is particularly robust. The MoS2 nanoribbons can readily be removed from its substrate and dispersed in solution. It is shown that functionalized MoS2 nanoribbons can be manipulated in solution and assembled in controlled patterns and directly on microelectrodes with UV-click-chemistry. Owing to the high chemical purity and polycrystalline nature, the MoS2 nanostructures demonstrate rapid optoelectronic response to wavelengths from 450 to 750nm, and successfully remove mercury contaminants from water. The scalable fabrication and manipulation followed by light-directed assembly of MoS2 nanoribbons, and their unique properties, will be inspiring for device fabrication and applications of the transition metal dichalcogenides.

Metastable (CuAu-type) CuInS2 Phase: High-Pressure Synthesis and Structure Determination.

We report on the high-pressure solid-state synthesis and the detailed structural characterization of the metastable, CuAu-type CuInS2 (CA-CIS) phase. Although often present in CIS thin films as unwanted phase, it has been never synthesized in pure form, and its effect on the performance of CIS-based solar cells has been long debated. In this work, pure CA-CIS phase is synthesized in bulk polycrystalline form through a high-pressure-high-temperature solid-state reaction. Single-crystal X-rays diffraction reveals the formation of tetragonal CA-CIS (a = 3.9324(5), c = 5.4980(7) Å) either in cation-ordered and disordered phase, pointing out the role of the pressure/temperature increase on the Cu/In ordering. The resistivity measurements performed on CA-CIS show low resistivity and a flat trend vs temperature and, in the case of the ordered phase, highlight a bad-metallic behavior, probably due to a high level of doping. These findings clearly rule out the possibility of a beneficial effect of this phase on the CIS-based thin film solar cells.

Nano- and Micro-Scale Simulations of Ge/3C-SiC and Ge/4H-SiC NN-Heterojunction Diodes

During the last decade, silicon carbide (SiC) and its heterostructures with other semiconductors have gained a significant importance for wide range of electronics applications. These structures are highly suitable for high frequency and high power applications in extremely high temperature environments. SiC exists in more than 200 different polycrystalline forms, called polytypes. Among these 200 types, the most prominent polytypes with exceptional physical and electrical attributes are 3C-SiC, 4H-SiC and 6H-SiC. Heterostructures of these SiC polytypes with other conventional semiconductors (like Si, Ge) can give rise to interesting electronic characteristics. In this article, Germanium (Ge) has been used to make heterostructures with 3C-SiC and 4H-SiC using a novel technique called diffusion welding. Microscale and nanoscale simulations of nn-heterojunction of Ge/3C-SiC and Ge/4H-SiC have been done. Microscale devices have been simulated with a commercially available semiconductor device simulator tool called Silvaco TCAD. Whereas nanoscale devices have been simulated with QuantumWise Atomistix Toolkit (ATK) software package. Current-voltage (IV) curves of all simulated devices have been calculated and compared. In nanoscale device, the effects of defects on IV-characteristics due to non-ideal bonding (lattice misplacement) at heterojunction interface have been analyzed. Our simulation results reveal that the proposed heterostructure devices with diffusion welding of wafers are theoretically possible. These simulations are the preparations of our near future physical experiments targeted to fabricate SiC based heterostructure devices using diffusion bonding technique.

The Emergence of an Itinerant Ferromagnetic State in Co‐Doped YNi5: A Critical Behavior Study of the Phase Transition

YNi5 is an enhanced Pauli paramagnetic material, and this work is devoted to the effect of Co doping at the Ni site of YNi5. It is found that at 10% Co doping, the sample turns ferromagnetic, with a weak itinerant character. The critical behaviours of the 10% Co‐doped sample (i.e., YNi4.5CO0.5) in its polycrystalline form are examined through bulk magnetization measurements. The second‐order phase transition from the paramagnetic to the ferromagnetic state is confirmed by the positive slope of the Arrott plots. To obtain the most reliable values of the critical exponents, an extensive analysis of the magnetization data is performed, which includes a modified Arrott plot, Kouvel–Fisher plot, and critical isotherm analysis. The best values of the exponent obtained are β = 0.179 (±0.002), γ = 1.429 (±0.002), and TC = 9.1 K. These values are close to a 2D Ising model along with the long‐range exchange decaying as J(r) ≈ r−(3.6). It is found that this extended exchange is a typical characteristic of many itinerant ferromagnets, whereas the 2D nature is likely to be connected with the layered crystal structure.

The Soft Molecular Polycrystalline Ferroelectric Realized by the Fluorination Effect.

For a century ferroelectricity has attracted widespread interest from science and industry. Inorganic ferroelectric ceramics have dominated multibillion dollar industries of electronic ceramics, ranging from nonvolatile memories to piezoelectric sonar or ultrasonic transducers, whose polarization can be reoriented in multiple directions so that they can be used in the ceramic and thin-film forms. However, the realization of macroscopic ferroelectricity in the polycrystalline form is challenging for molecular ferroelectrics. In pursuit of low-cost, biocompatible, and mechanically flexible alternatives, the development of multiaxial molecular ferroelectrics is imminent. Here, from quinuclidinium perrhenate, we applied fluorine substitution to successfully design a multiaxial molecular ferroelectric, 3-fluoroquinuclidinium perrhenate ([3-F-Q]ReO4), whose macroscopic ferroelectricity can be realized in both powder compaction and thin-film forms. The fluorination effect not only increases the intrinsic polarization but also reduces the coercive field strength. More importantly, it is also, as far as we know, the softest of all known molecular ferroelectrics, whose low Vickers hardness of 10.5 HV is comparable with that in poly(vinylidene difluoride) (PVDF) but almost 2 orders of magnitude lower than that in BaTiO3. These attributes make it an ideal candidate for flexible and wearable devices and biomechanical applications.

Establishing magneto-structural relationships in the solid solutions of the skyrmion hosting family of materials: GaV4S8\u2212ySey

The GaV4S8−ySey (y = 0 to 8) family of materials have been synthesized in both polycrystalline and single crystal form, and their structural and magnetic properties thoroughly investigated. Each of these materials crystallizes in the Fbar{4}3m space group at ambient temperature. However, in contrast to the end members GaV4S8 and GaV4Se8, that undergo a structural transition to the R3m space group at 42 and 41 K respectively, the solid solutions (y = 1 to 7) retain cubic symmetry down to 1.5 K. In zero applied field the end members of the family order ferromagnetically at 13 K (GaV4S8) and 18 K (GaV4Se8), while the intermediate compounds exhibit a spin-glass-like ground state. We demonstrate that the magnetic structure of GaV4S8 shows localization of spins on the V cations, indicating that a charge ordering mechanism drives the structural phase transition. We conclude that the observation of both structural and ferromagnetic transitions in the end members of the series in zero field is a prerequisite for the stabilization of a skyrmion phase, and discuss how the absence of these transitions in the y = 1 to 7 materials can be explained by their structural properties.

Low temperature deformation mechanisms of single crystalline intermetallic compound YAg

YAg belongs to the family of B2-type rare earth intermetallic compounds that in polycrystalline form exhibit moderate low temperature ductility. In this work, YAg single crystals have been deformed at low temperatures down to 4 K. The activation parameters and their dependence on stress and temperature, as well as the comparison with the deformation behavior of polycrystalline YAg, indicate that the cutting of forest dislocations is the rate-controlling deformation mechanism, similar to face-centered cubic metals. The results suggest that the low yield stress and high work-hardening rate observed cause the moderate ductility at low temperatures.

Phase equilibria, thermodynamic analysis and electrical properties of the Li2O–Y2O3–B2O3 system

Abstract Phase equilibria in the Li2O–Y2O3–B2O3 system are studied via the methods of thermodynamic, differential thermal analysis and X-ray diffractometry. The physico-chemical characteristics of the previously mentioned intermediate compounds with the compositions 2Li2O⋅Y2O3⋅5B2O3, 6Li2O⋅Y2O3⋅3B2O3, 3Li2O⋅2Y2O3⋅3B2O3 are clarified. T–x diagrams of polythermal sections, isothermal section at 25 °C, and liquidus surface projection for the Li2O–Y2O3–B2O3 system were constructed. The temperature dependencies for electrical conductivity of compounds of the Li2O–Y2O3–B2O3 system are studied. It has been shown that the conductivity of these compounds in polycrystalline form exponentially increases with increasing temperature. This indicates the semiconductor nature of the conductivity of the studied samples. The activation energies of the conductivity for these samples are determined.

Role of Potassium Substitution in the Magnetic Properties and Magnetocaloric Effect in La0.8−xKxBa0.05Sr0.15MnO3 (0 ≤ x ≤ 0.20)

The magnetic and magnetocaloric effects of potassium-substituted La0.8−xKxBa0.05Sr0.15MnO3 (0 ≤ x ≤ 0.20) manganite were explored. The samples in polycrystalline form were synthesized by the sol–gel method, with a final sintering temperature of 1100 °C. Powder X-ray diffraction (XRD) patterns refined by Rietveld refinement show that all samples crystallized in rhombohedral structure with R-3c space group. The unit cell volume of the samples decreases with increasing potassium concentration. In addition, small changes in average bond length and bond angle are also observed in the samples. Scanning electron microscope (SEM) images reveal that the largest average grain size was observed for x = 0.10. Field-cooled (FC) magnetization measurements show that the Curie temperature ( T C ) of the samples increases from 320 K for x = 0 to 360 K for x = 0.2. The largest magnetocaloric (MCE) effect, which is represented by maximum magnetic entropy change (− Δ S M , M A X ), reaches its greatest value for the x = 0.10 sample. The monotonous increase in T C suggests that TC is mainly governed by the ferromagnetic coupling between Mn ions induced by the changes on average bond length and bond angle. The obtained − Δ S M , M A X value suggests that MCE property is more sensitive to Zener theory of double exchange, which is strongly related to the Mn3+/Mn4+ ratio of the samples.

Increased electrical conduction with high hole mobility in anti-ThCr2Si2-type La2O2Bi via oxygen intercalation adjacent to Bi square net

Anti-ThCr2Si2-type RE2O2Bi (RE = rare earth) with a Bi square net is known to show an insulator–metal transition by substituting RE. In this study, La2O2Bi polycrystals with different oxygen nonstoichiometry were synthesized. As the amount of oxygen in La2O2Bi increased, the c-axis length was expanded due to the generation of an additional 4e site for excess oxygen, while the a-axis length remained almost constant, indicating the separation of Bi square nets by oxygen intercalation. Concomitantly, transformation of insulating La2O2Bi into metallic La2O2Bi occurred with the change in carrier polarity from the n- to p-type. Despite its polycrystalline form, La2O2Bi with the largest amount of oxygen showed a rather high hole mobility of 85 cm2 V−1 s−1 among other layered oxypnictides and oxychalcogenides.

Magnetic and transport anomalies inR2RhSi3(R=Gd, Tb, and Dy) resembling those of the exotic magnetic materialGd2PdSi3

We have carried out magnetization, heat capacity, electrical and magnetoresistance measurements (2-300 K) for the polycrystalline form of intermetallic compounds, R2RhSi3 (R= Gd, Tb, and Dy), forming in a AlB2 derived hexagonal structure with a triangular R network. This work was primarily motivated by a revival of interest on Gd2PdSi3 after about two decades in the field of Toplogical Hall Effect due to magnetic skyrmions. We report here that these compounds are characterized by double antiferromagnetic transitions (T_N= 13.5 and 12 K for Gd, 13.5 and 6.5 K for Tb; 6.5 and 2.5 for Dy), but antiferromagnerism seems to be complex. The most notable observations common to all these compounds are: (i) There are many features in the data mimicking those seen for Gd2PdSi3, including the two field-induced changes in isothermal magnetization as though there are two metamagnetic transitions well below T_N. In view of such a resemblance of the properties, we speculate that these Rh-based materials offer a good playground to study toplogical Hall effect in a centrosymmetric structure, with its origin lying in triangular lattice of magnetic R ions; (ii) There is an increasing contribution of electronic scattering with decreasing temperature towards T_N in all cases, similar to Gd2PdSi3, thereby serving as examples for a theoretical prediction for a classical spin-liquid phase in metallic systems due to geometrical frustration.

AlN and MgO thin-layer coatings on the bendable polymeric substrates as selective filters for IR and THz spectral ranges

Thin AlN and MgO poly- or nano-structured layers were synthesized on bendable polymeric substrates using the helicon-arc ion-plasma method. They can be used as filters to block infrared (IR) radiation from civil and military objects. To suppress IR radiation within the range λ ~ 7.5…25 µm, where most of the radiation power from objects with the temperature T ~ 300…400 K is concentrated, we have used AlN and MgO thin layers (d ~ 1.5…10 μm) that have the Restrahlen bands within the spectral range indicated above. The optical, structural properties and chemical composition of AlN and MgO films on the Mylar or Teflon substrates have been investigated. These filters demonstrate blocking characteristics for masking the thermal image of objects heated to a temperature of 300…400 K in the IR spectral range. At the same time, they are well transparent in the microwave and THz spectral ranges (transparency τ up to ≈ 80…85%). Moreover, thin Mylar films with AlN coatings are transparent in the visible range. Using the combined coatings of AlN with high thermal conductivity (110…140 W/m·K in the polycrystalline form) for heat withdraw to the periphery or air and MgO coatings, one can suppress IR radiation by more than 90% within the spectral range λ = 7.5…25 μm.

Antiferromagnetic metallic state as proved by magnetotransport in epitaxially stabilized perovskite PbRuO3

Perovskite ${\mathrm{PbRuO}}_{3}$, which has been synthesized only in bulk polycrystalline form under high pressure, is an orthorhombic Pbnm at room temperature, exhibiting structural phase transition to Imma at around 90 K. This structural transition is accompanied with an orbital ordering and antiferromagnetic spin fluctuation. Here, we report the fabrication of single crystalline perovskite ${\mathrm{PbRuO}}_{3}$ thin films on various substrates. Magnetotransport measurements reveal that the films have metallic electronic ground state without any clear electronic transitions reported in previous literature. Evidenced by a shift of transition temperature, suppression of magnetic spin fluctuation due to epitaxial strain is implicated. Nonlinear Hall effect is also observed with indiscernible hysteresis, plausibly originating from antiferromagnetic spins, yet multiband effect cannot be completely ruled out.

Template-free Synthesis of Mesoporous and Crystalline Transition Metal Oxide Nanoplates with Abundant Surface Defects

Summary Mesoporous transition metal oxides with crystalline walls show promising applications in catalysis and energy storage. Syntheses of those crystalline mesoporous oxides, however, currently remain as an unmet challenge because of the large volume shrinkage during crystallization in a sol-gel process. Here, we demonstrate a facile and general template-free method to prepare six mesoporous transition metal oxides, e.g., CuO, CoO, and spinel MCo2O4 (M = Co, Cu, Mn, Zn), with highly crystalline walls and continuous mesopores in the forms of two-dimensional nanoplates or nanosheets. The method is based on the thermal crystalline transformation from basic carbonates to mesoporous metal oxides. The crystal interconversion not only endows the crystallinity and mesoporosity of oxides but also generates abundant surface-step defects that show remarkable enhancement of the catalytic activity of porous oxides. Our method likely provides an alternative paradigm for cost-effective and universal synthesis of crystalline mesoporous oxides beyond the traditional templating methods.