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Laser Heated Diamond Anvil Cell facility

for Synthesis of Novel Materials

In the recent decade the Laser Heated Diamond Anvil Cell (LHDAC) technique has become the preferred tool to studymaterials at extremely high pressures andtemperatures on account of (a)accessibility to Pressure (P) and Temperature (T)ranges not possible by conventional methods; (b) ultrapure sample environment; (c) highly localized heating of the sample.Applications of the LHDAC include exploration of P-Tphase diagrams of materials, search for novel phases like superhardand harder-than-diamond materials, simulation of planetary interiors etc. Keeping some of these in mind, a state of-the-artLHDAC facility hasbeen set-upsuccessfully to study materials under very high pressures(~ 1 Mbar) andtemperatures(~ 5000 K). There are some unique features in this facility, like a fully automated scan routine to achieve uniform arealheating of the sample using nanomotion systems and an indigenous imaging arrangement.

The Diamond Anvil Cell (DAC) is a tool par excellence tocompress matter to very high static pressures. High strengthof the diamonds coupled with their excellent transmittanceto almost the entire electromagnetic spectrum has led toextensive in situ studies on a myriad of pressure inducedphenomena in materials using X-ray diffraction and variousspectroscopic techniques. This wide-range transmittance ofdiamonds can be exploited to focus high power IR laserbeams on to the sample squeezed in the DAC, therebysubjecting it simultaneously to very high temperatures ofover 5000 K and pressures of over many megabars. Thistechnique, called the Laser Heated Diamond Anvil Cell(LHDAC) is emerging as an unique route for investigatingmaterials and synthesizing novel phases in the hithertounexplored P-T region. The LHDAC technique can be utilizedfor simulating, in the laboratory, the extreme environmentssuch as those existing in Earth andother planetary interiors.At such extreme conditions, nature of the chemical bondingin elements changes and the chemical reactivities generallyincrease such that direct elemental reaction between eveninert species becomes possible. This can lead to theformation of novel and exotic phases. Synthesis of nitrides ofC, B, Si and Ge that are expected to rival diamond in terms ofhardness is one of the frontline problems addressed byLHDAC researchers. Other applications of the LHDACinclude study of melting phenomena and structuraltransitions in a variety of materials.

At Materials Science Division, IGCAR, a state-of-the-artLHDAC facility has been set-up recently for investigatingmaterials behaviour up to ~1 mbar and ~5000 Ksimultaneously (Figs. 1 and 2). This facility essentiallyconsists of an indigenously developed DAC, a 120 W CO

laser ( =10.6 m; continuous wave; single mode), a X-YNanomotion system, and IR compatible optical components.In this technique, the challenge lay in heating the ultra tinysample(~75 m diameter) that is squeezed between the twodiamond anvils to very high-temperatures using an invisiblehigh powerIR laser without causing damage to thediamonds

and other supporting materials. This has been achievedu si ng s im pl e a nd c os t e ff ec ti ve i de as l ik e awebcameramicroscope combination for real-time imaging(now, upgraded with a CCD imaging system), indigenouslydesigned vibration free tables and optical mounts. The use ofthe nanomotion system, for which the control software wasdeveloped inhouse, played a critical role in achieving theabove successfully. A CCD based spectrometer has beenincluded in the set-up to (a) record the grey body spectrumemitted by the sample for temperature estimation usingPlanck distribution; and (b) to record luminescence from aruby chip, located inside the DAC, excited by 514.5 nm linefrom an argon ion laser for pressure determination.

Valuable experience in sample synthesis at high-pressuresand high-temperatures have been gained using this IGCARLHDAC facility. In particular, two experiments thatdemonstrated the capability of this facility and mastery overthe difficult technique are conversion of graphite to adiamond-like translucent phase at ~14 GPa and ~2000 Kand successful synthesis of the semiconducting materialInSbby direct heating of Inand Sbat ~2000Kand ~0.5GPa.

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Fig.1 : Schematic of the LHDAC set-up; M1, M2, M3and M4: Mirrors;BS: Beam Steerer; BX: Beam Expander; FL1 and FL2:

Focussing Lenses; CL: CollectingLens;

Fig. 2 : Photograph of the LHDAC set-up; All components, includingthelasers aremounted on a granite table located on a special

foundation, structurally isolated from the rest of the buildingto minimize vibrations.

n OUTLINE

n EXECUTIVE SUMMARY

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It is well known that diamond is a metastable phase of carbon and canbe synthesized, even without the aid of any catalyst, in the laboratoryby subjecting carbon in one of its myriad forms like graphite orfullerene to very high pressures and temperatures. This experimentwas taken up as the first problem in order to test the IGCAR LHDACfacility and gain experience in handling the difficult LHDACtechnique.

A high purity graphite chip (linear dimension ~75 m), sandwichedbetween two KBr layers was loaded in the DAC (Fig.3). The KBr layerswere used to thermally insulate the sample from the diamond anvilsas also provide optical windows. At a pressure of ~14 GPa and at a CO

laser power of ~15 Watts, exothermic reaction characterised byflashes of very bright light got initiated. The temperature wasestimated to be ~2000 K. Using the nanomovers, the sample wasrastered by the CO laser beam. After the heating event lasting for

nearly 45 minutes, it was seen that the entire graphite chip hadbecome translucent to light, indicating formation of a new diamond-like translucent phase(Fig.3). Photographs at the bottom depict viewsof the graphite sample through the table of one of the diamond anvils.The three photos correspond to the state of the sample before heating,during laser heating and after laser heating. It can be seen that afterlaser heating, the sample has become translucent to light.

In Fig.3, the red beam represents the heating CO laser (10.2 m) whilethe green beam represents the argon ion laser (514.5 nm) used toexcite ruby luminescence for pressure measurement.

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Fig. 3 : Illustration of a typical sample assembly for LHDACexperiments and webcamera pictures showing

h eati ng of graphite at h igh pressur e and

temperature using IGCAR LHDAC

DAC is a device used to generate very high pressures on materials. It essentially consists of two identical gem qualitydiamonds shapedin the form of anvils andaligned perfectly (Fig.3). One of them is mounted on to a piston and theother on

to a cylinder. A pre-indented metal gasket, with a 200 m (dia) X 75 m (depth) hole to act as sample chamber, is spacedbetween the two culets (the flat polished tip of the diamond is called the culet). Very high pressure is generated on thesample when the piston is moved against the cylinder using an appropriate arrangement. IGCAR DACs have beendesigned to do high-pressure experiments up to a maximum of 1 million atmospheres. To get a feel for the pressure, it maybe noted that thepressure at theEarth's core is about3.5 million atmospheres.

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Very few laboratories across the world have mastered the LHDAC technique, owing to the intricacies involved and risks offrequent, irreversible damage to the diamond anvil cells (DAC). In IGCAR, several LHDAC heating experiments, eachlasting for few hours have now been done. Careful inspection of the DAC has not revealed any damage to the cell or itsconstituents, demonstrating the high-level of perfection in the optics that has been designed and set-up. With thissuccess, IGCAR is looking forward to establishing a similar facility centered around the XRD beamline at INDUS-IIsynchrotron (Indore). The published report on IGCAR LHDAC has attracted widespread attention and has receivedcompliments from eminent scientists from within the country and abroad.

Further inquiries:Dr. N. Subramanian, Materials Science Division

Metallurgy & Materials Group, IGCAR, e-mail: subbi@igcar.gov.in

N. S ubramanian, N . V . Chandra S hekar, N . R . Sanjay K umar a nd P . Ch. S ahu , , 2006)Current Science 91 ( 175.

Apart from expected changes in the unit cell volume, dictated by the Equation of State of the material under study, high-pressure affects the material in principally two ways, namely induction of structural and electronic changes. Structuralchanges include increased densification and phase transitions from one structure to another, generally involving changein crystallographic symmetry. Electronic changes result from increasing overlap of the electronic orbitals, leading to

transitions like metal-insulator, normal-superconducting, magnetic-nonmagnetic, etc. Expectedly, structural andelectronic effects are often interrelated and influence each other. Simultaneous application of high temperature enhancesthe kinetics and brings about interesting interplays with the pressure induced perturbation of the electronic and crystalstructures, resulting in novel ground states of the material.

During laser heating

Graphite at ~14 GPa

Before laser heating

P ~14 GPa;T ~2000 K

After laser heating

n EFFECTS OF PRESSURE AND TEMPERATURE ON MATERIALS BEHAVIOR

n PHASE TRANSITION OF GRAPHITE TO DIAMOND-LIKE MATERIAL

n WHAT IS A DIAMOND ANVIL CELL (DAC)?

n ACHIEVEMENT

n PUBLICATIONS ARISING OUT OF THIS STUDY AND RELATED WORK

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