Instrumentation development for magnetic and structural studies under extremes of pressure and temperature

The study of the magnetic and structural properties of matter under extreme conditions is a fast developing field. With the emergence of new techniques and innovative instruments for measuring physical properties, the need for compatible pressure generating devices is constantly growing. The work described in this thesis is focused on development, construction and testing of several high pressure (HP) cells of novel design. One of the cells is intended for single crystal X-ray diffraction (SXD) studies at low temperature (LT) and the other three HP devices are designed for a Magnetic Property Measurement System (MPMS), two of which are suitable for dc susceptibility studies and the other one is aimed at high frequency ac susceptibility measurements. HP crystallographic studies are routinely carried out in diamond anvil cells (DAC) at room temperature while ambient pressure SXD studies are often conducted at LT to reduce atomic vibrations and obtain more precise structural data as well as to study LT phases. Combining HP with LT gives access to a whole new area on the phase diagrams but due to the size of the existing DACs this is generally achieved by cooling down the cells inside a cryostat and it is mainly possible at synchrotrons where dedicated facilities exist. A miniature DAC which can be used with commercially available laboratory cry-flow cooling systems and achieves pressures in excess of 10 GPa has been developed. The design of the pressure cell is based on the turnbuckle principle and therefore it was called TX-DAC. Its dimensions have been minimised using Finite Element Analysis (FEA) and the final version of the cell weighs only 2.4 g. The cell is built around a pair of 600 μm culet Boehler-Almax anvils which have large conical openings for the diffracted beam. The TX-DAC is made of beryllium copper (BeCu) alloy which has good thermal conductivity and allows quick thermal equilibration of the cell. The MPMS from Quantum Design is the most popular instrument for studies of magnetic properties of materials. It is designed to measure ac and dc magnetic susceptibility of sample with detectable signals as low as 10-8 emu. The MPMS has a sample chamber bore of 9 mm in diameter and this puts a constraint on the dimensions of the pressure cells. However, several types of clamp piston-cylinder cells and DACs have been designed for the MPMS. The former are used for measurements at pressure up to 2 GPa and the later can be used for studies at higher pressure. Taking advantage of the turnbuckle principle, a DAC (TM-DAC) and a piston-cylinder cell (TM-PCC) for dc magnetic studies were built. They allow HP measurements to be performed at the full sensitivity of MPMS. Both pressure cells are made of BeCu and their small dimensions combined with symmetrical design is the key to an ideal background signal correction. The TM-DAC is 7 mm long and 7 mm in diameter, it weighs 1.5 g and with 800 μm culet anvils it can generate a sample pressure of 10 GPa. Inherently the sample volume is limited to approximately 10-3 mm3 and the signal corresponding to this volume of some weakly magnetic material remains below the sensitivity of the MPMS. This constraint led us to the development of the TM-PCC – a piston-cylinder variant of the turnbuckle design. With a 4 mm3 sample volume it allows the study of weakly magnetic samples in the range 0-1.9 GPa. The TM-PCC uses two zirconia pistons of 2.5 mm in diameter; it is 10 mm long, 7 mm in diameter and weights 2.7 g. Conventional metallic pressure cells perform well in dc mode however in ac susceptibility measurements, the Eddy currents set in the cells’ body lead to a screening effect which can significantly obscure the signal from the sample. This problem was solved by designing a composite piston-cylinder cell made with Zylon fibre and epoxy resin. The sample is located in the middle of the cell in the 2.5 mm bore and the pressure is transmitted through zirconia pistons. Keeping the metallic parts away from the sample resolves any interference issue. The composite cell performs well in a pressure range of 0-1 GPa. The performance of the pressure cells developed within this project is illustrated by studies of various systems at high pressure

Turnbuckle diamond anvil cell for high-pressure measurements in a superconducting quantum interference device magnetometer

We have developed a miniature diamond anvil cell for magnetization measurements in a widely used magnetic property measurement system commercial magnetometer built around a superconducting quantum interference device. The design of the pressure cell is based on the turnbuckle principle in which force can be created and maintained by rotating the body of the device while restricting the counterthreaded end-nuts to translational movement. The load on the opposed diamond anvils and the sample between them is generated using a hydraulic press. The load is then locked by rotating the body of the cell with respect to the end-nuts. The dimensions of the pressure cell have been optimized by use of finite element analysis. The cell is approximately a cylinder 7 mm long and 7 mm in diameter and weighs only 1.5 g. Due to its small size the cell thermalizes rapidly. It is capable of achieving pressures in excess of 10 GPa while allowing measurements to be performed with the maximum sensitivity of the magnetometer. The performance of the pressure cell is illustrated by a high pressure magnetic study of Mn(3)[Cr(CN)(6)](2)center dot xH(2)O Prussian blue analog up to 10.3 GPa. (C) 2010 American Institute of Physics. [doi:10.1063/1.3465311]</p

Effect of Thermal Cycling on the Structural Evolution of Methylammonium Lead Iodide Monitored around the Phase Transition Temperatures

Optoelectronic devices and solar cells based on organometallic hybrid perovskites have to operate over a broad temperature range, which may contain their structural phase transitions. For instance, the temperature of 330 K, associated with the tetragonal-cubic transformation, may be crossed every day during the operation of solar cells. Therefore, the analysis of thermal cycling effects on structural and electronic properties is of significant importance. This issue is addressed in the case of methylammonium lead iodide (CH3NH3PbI3) across both structural phase transitions (at 160 and 330 K). In situ synchrotron radiation X-ray diffraction (XRD) data recorded between 140 and 180 K show the emergence of a boundary phase between the orthorhombic and tetragonal phases, which becomes more abundant with successive thermal cycles. At high temperatures, around 330 K, an incommensurately modulated tetragonal phase is formed upon repeated crossings of the phase boundary between tetragonal and cubic phases. These alterations, which indicate a gradual evolution of the material under operating conditions of photovoltaic devices, are further documented by electrical resistivity and heat capacity measurements

In Situ Comparison of Ionothermal Kinetics Under Microwave And Conventional Heating

We have used in situ energy dispersive synchrotron X-ray diffraction to study the crystallization of aluminum phosphate frameworks under ionothermal conditions with conventional and microwave heating. The reaction is shown to follow slightly different routes depending on the type of heating used and a kinetic analysis shows that the rate constant is 10 times higher under microwave heating (1.4 compared to 0.14 min(-1)). The conventionally heated reaction is shown to proceed by a transformation of SIZ-3 to SIZ-4 via an intermediate, while the microwave-heated reaction forms SIZ-4 directly. The kinetic analysis is used to rationalize the differences in reaction rate and the findings are supported by SEM images of the crystal morphologies which result from the two heating methods.</p

Pressure-induced transformation of CH3NH3PbI3: the role of the noble-gas pressure transmitting media

The photovoltaic perovskite, methylammonium lead triiodide [CH3NH3PbI3 (MAPbI(3))], is one of the most efficient materials for solar energy conversion. Various kinds of chemical and physical modifications have been applied to MAPbI(3) towards better understanding of the relation between composition, structure, electronic properties and energy conversion efficiency of this material. Pressure is a particularly useful tool, as it can substantially reduce the interatomic spacing in this relatively soft material and cause significant modifications to the electronic structure. Application of high pressure induces changes in the crystal symmetry up to a threshold level above which it leads to amorphization. Here, a detailed structural study of MAPbI(3) at high hydrostatic pressures using Ne and Ar as pressure transmitting media is reported. Single crystal X-ray diffraction experiments with synchrotron radiation at room temperature in the 0-20 GPa pressure range show that atoms of both gaseous media, Ne and Ar, are gradually incorporated into MAPbI(3), thus leading to marked structural changes of the material. Specifically, Ne stabilizes the high-pressure phase of Ne(x)MAPbI(3) and prevents amorphization up to 20 GPa. After releasing the pressure, the crystal has the composition of Ne(0.97)MAPbI(3), which remains stable under ambient conditions. In contrast, above 2.4 GPa, Ar accelerates an irreversible amorphization. The distinct impacts of Ne and Ar are attributed to differences in their chemical reactivity under pressure inside the restricted space between the PbI6 octahedra