Muhammad Adnan Asghar

Beryllium-Free Rb3Al3B3O10F with Reinforced Interlayer Bonding as a Deep-Ultraviolet Nonlinear Optical Crystal

A new beryllium-free borate Rb3Al3B3O10F has been synthesized and characterized by single-crystal X-ray diffraction. It features a framework structure consisting of alveolate [Al3(BO3)3OF]∞ layers tightly bound via Al-O and Al-F bridged bonds, with the in-layer [BO3](3-) groups in nearly coplanar and aligned arrangement. This compound is transparent down to 200 nm and is phase-matchable with a powder second-harmonic generation efficiency of 1.2 times that of KH2PO4. Remarkably, it exhibits a strong interlayer bonding which is about one order larger than that of the benchmark KBe2BO3F2, thus no layering tendency was observed during the crystal growth. In addition, it is nonhygroscopic and thermally stable up to ∼1462 K. These attributes make Rb3Al3B3O10F a promising nonlinear optical crystal in the deep-ultraviolet region. First-principles calculations, combined with the anionic group theory, were adopted to rationalize the optical properties.

Designing a Beryllium-Free Deep-Ultraviolet Nonlinear Optical Material without a Structural Instability Problem

A beryllium-free deep-ultraviolet (deep-UV) nonlinear optical (NLO) material K3Ba3Li2Al4B6O20F is developed mainly by the element substitution of Be for Al and Li from Sr2Be2B2O7 that was considered as one of the most promising deep-UV NLO materials. K3Ba3Li2Al4B6O20F preserves the structural merits of Sr2Be2B2O7 and thus exhibits no layering growth tendency and possesses the optical properties required for deep-UV NLO applications, including deep-UV transparency, phase-matchability, and sufficiently large second-harmonic generation (1.5 × KH2PO4). Furthermore, it overcomes the structural instability problem of Sr2Be2B2O7, which is confirmed by the obtainment of large single crystals and phonon dispersion calculations. These attributes make it very attractive for next-generation deep-UV NLO materials. The substitution of Be for Al and Li in beryllium borates provides a new opportunity to design beryllium-free deep-UV NLO materials with good performance.

A Photoferroelectric Perovskite-Type Organometallic Halide with Exceptional Anisotropy of Bulk Photovoltaic Effects.

Perovskite-type ferroelectrics composed of organometallic halides are emerging as a promising alternative to conventional photovoltaic devices because of their unique photovoltaic effects (PVEs). A new layered perovskite-type photoferroelectric, bis(cyclohexylaminium) tetrabromo lead (1), is presented. The material exhibits an exceptional anisotropy of bulk PVEs. Upon photoexcitation, superior photovoltaic behaviors are created along its inorganic layers, which are composed of corner-sharing PbBr6 octahedra. Semiconducting activity with remarkable photoconductivity is achieved in the vertical direction, showing sizeable on/off current ratios (>10(4) ), which compete with the most active photovoltaic material CH3 NH3 PbI3 . In 1 the temperature-dependence of photovoltage coincides fairly well with that of polarization, confirming the dominant role of ferroelectricity in such highly anisotropic PVEs. This finding sheds light on bulk PVEs in ferroelectric materials, and promotes their application in optoelectronic devices.

Reversible phase transition driven by order–disorder transformations of metal-halide moieties in [(C6H14)NH2]2·CuBr4

A novel phase transition has been discovered where the phase transition is primarily accomplished by the order–disorder transformation of metal-coordinated halogen atoms in an organic–inorganic hybrid material [(C6H14)NH2]2·CuBr4 (1). Its phase transition behaviour was verified by specific heat capacity (Cp) and differential scanning calorimetry (DSC) measurements with a thermal hysteresis at 4.8 K. The dielectric measurements of 1 show a distinct step-like anomaly around Tc, demonstrating two states at two different phases, which enlightens that 1 can be conceived as a potential switchable dielectric material. Moreover, temperature-dependent single-crystal X-ray diffraction analyses of 1 disclose that disordering of metal-halides together with reorientations of the cations is the cause of this unique phase-transition material. All these results open a new way to design and assemble novel phase transition materials.

Order–disorder phase transition coupled with torsion in tri-n-butylammonium trichloroacetate (TBAT)

A novel molecular phase transition compound, tri-n-butylammonium trichloroacetate (TBAT), has been successfully synthesized with a reversible phase transition at 196 K. Its phase transition behaviour was confirmed by specific heat capacity and differential scanning calorimetry (DSC) measurements with a 2 K thermal hysteresis, indicating that the phase transition is a first-order one. Dielectric measurements further reveal the reversible nature of the phase transition, which exhibits distinctive step-like anomalies between low and high dielectric states. Moreover, temperature dependent single-crystal X-ray diffraction analyses of TBAT disclose that the order–disorder transformation of the flexible tri-n-butylammonium cations and the haloacetic acids anion together with the torsion in the cations stimulate the structural phase transition. All these results open a new way to design and assemble novel phase transition materials.

A supra-molecular switchable dielectric material with non-linear optical properties

A novel molecular phase transition material (PTM), 1-methylpiperidinium perchlorate [18-crown-6] monohydrate (1), has been synthesized, which exhibits reversible switchable dielectric anomalies near room temperature. The phase transition in 1 is from noncentrosymmetric to noncentrosymmetric, displaying non-linear optical (NLO) properties with a second harmonic generation response of ∼0.8 times compared to that of potassium dihydrogen phosphate. Thermal analyses of 1, including differential scanning calorimetry and specific heat measurements, confirm the first-order solid-state phase transition at 260 K. The dielectric constants display temperature-dependent anomalies with the temperature approaching the transition point (Tc), where the evident step-like anomalies demonstrate two distinct states below and above the Tc value, respectively. Variable-temperature single crystal X-ray diffraction discloses that the origin of its phase transition is ascribed to the order–disorder transformation of perchlorate anion, the methyl group of cation and the torsional angular change in crown molecule; that is, all three components contribute to the emergence of this phase transition. This result suggests that 1 could be conceived as a potential switchable dielectric and NLO material, which provides an effective approach to design novel NLO switch materials.

A High‐Temperature Order–Disorder Phase Transition Coupled With Conformational Change in the Hybrid Material [C6H13NH]2⋅ZnBr4

A new high-temperature, hybrid, phase-transition material, 1-methylpiperidinium tetrabromozincate (1), that shows a reversible transition at 345 K was synthesized. Differential scanning calorimetry and specific heat capacity measurements confirmed this reversible transformation with a large heat hysteresis of 25 K, which describes a typical first-order phase transition in 1. The dielectric constant exhibited a steplike anomaly and showed high and low dielectric states in the high- and room-temperature phases, respectively, and therefore, this hybrid might be considered as a potential switchable dielectric material. The variable-temperature powder X-ray diffraction patterns displayed remarkable shifts between the experimental patterns at the two different phases. Single-crystal X-ray diffraction analyses at various temperatures revealed that the origin of this transformation could be attributed to disordering of the bromine atoms in the anion and the nitrogen atom of the cation. The cation also assumed a conformational change, which was likely induced by the disordered nitrogen atom. The conformational onset of the transformation of the cation from a planar conformer into a relaxed chair also occurred upon decreasing the temperature below transition point; thus, the combined order-disorder and conformational change induced the structural transformation and the change in symmetry.

[C5H12N]CdCl3: an ABX3 perovskite-type semiconducting switchable dielectric phase transition material

A new organic–inorganic ABX3 perovskite-type semiconducting hybrid, N-methylpyrrolidinium trichlorocadmate (MPCC), has been synthesized. It is found that MPCC exhibits semiconducting behaviour with an optical bandgap of ∼4.65 eV, as well as remarkable dielectric switching properties. It displays a structural phase transition (SPT) at Tc = 254 K, being confirmed by DSC and specific heat measurements. Dielectric measurements display distinct step-like anomalies around Tc, suggesting MPCC as a switchable dielectric material. Variable temperature single-crystal X-ray diffraction reveals that symmetry-breaking occurs from Pnma (at 280 K, >Tc) to P21/c (at 230 K, <Tc), which is attributed to the ordering of the disordered N-methylpyrrolidinium (MP) cation. The most distinct difference between its low- and high-temperature structures is the order–disorder dynamics of the MP cation, which mainly contribute to the SPT and switchable dielectric activities of MPCC. This finding paves a new way for designing multi-featured ABX3-type SPT hybrids by choosing disordered cationic moieties.

N-Methylpyrrolidinium hydrogen tartrate (NMPHT): an above-room-temperature order–disorder molecular switchable dielectric material

A novel molecular switchable dielectric material N-methylpyrrolidinium hydrogen tartrate (NMPHT) has been synthesized, which undergoes above room temperature phase transition. Thermal measurements, e.g. differential scanning calorimetry and specific heat, confirm the presence of a phase transition around 317.5 K. The dielectric measurement displays a distinct step-like anomaly around Tc, showing two different dielectric states, which reveals that NMPHT is a switchable dielectric material. The single-crystal X-ray diffraction analyses at variable temperatures demonstrate that the phase transition emerges from the severe disordering of the N-methylpyrrolidinium (NMP) cation, and during the transition the symmetry of NMPHT is transformed from higher (C2/c) to lower (P21/n). These findings will provide a new horizon to design smart dielectric structural phase transition materials by incorporating a flexible NMP based scaffold.

Temperature-triggered order–disorder phase transition in molecular-ionic material N-butyldiethanolammonium picrate monohydrate

A new molecular-ionic phase transition material, N-butyldiethanolammonium picrate monohydrate (BEAPM), which exhibits reversible switchable dielectric performances, has been successfully assembled. This compound undergoes a first-order solid-state phase transition at 160 K (Tc), which is confirmed by the thermal analyses including differential scanning calorimetry (DSC), specific heat (Cp) and dielectric measurements. Variable-temperature single crystal X-ray diffraction reveals that the origin of its phase transition is ascribed to the order–disorder transformation of oxygen atoms of the poly-nitro aromatic system, i.e. the picrate anions. Interestingly, the dielectric constants display clear temperature-dependant anomalies with the temperature approaching to Tc. The evident step-like anomalies of dielectric constants demonstrate two distinct states below and above the Tc, respectively. This result signifies that BEAPM could be conceived as a potential switchable dielectric material. These findings make us believe that BEAPM might be a potential solid–solid dielectric phase transition material.