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Polev, A. V.; Mikhin, N. P.; Rudavskiı̆, É. Ya.

doi:10.1023/a:1015243118083

Cited by 4 publications

(6 citation statements)

References 5 publications

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“…7 A number of authors have conjectured the possible remelting of the initial crystal in the process of the structural phase transition. 3,4,6,7 An additional diffusion process with an anomalously high value of the diffusion coefficient, close to the corresponding value for liquid helium, has also been registered. 3,4 New features of the bcc-hcp transition have been established 2,6 in the region of the upper and lower triple points on the melting curve of 4 He.…”

Section: Introductionsupporting

confidence: 52%

“…This cycle of experimental studies of the kinetics of the bcc-hcp transition in 4 He under different conditions has revealed a number of new features.…”

Section: Discussionmentioning

confidence: 99%

“…In this study we have investigated 4 He crystals with a molar volume of 20.85-21.10 cm 3 / mole, which corresponded to pressures of 25-31 bar temperatures of 1.25-1.90 K.…”

Section: Experimental Techniquementioning

confidence: 99%

“…On the P -T phase diagram of 4 He the bcc phase occupies a narrow region ͑not more than 50 mK wide͒ near the melting curve. Within this region a change of temperature by ⌬T usually leads to an exponential variation of the pressure in the sample with time ͑Fig.…”

Section: A the Bcc Phasementioning

confidence: 99%

“…Experiments in recent years 1-7 have established a number of unexpected features in the properties of solid 4 He and dilute mixtures of 3 He in 4 He near the bcc-hcp transition: anomalous behavior of the diffusion coefficient, specific structural changes in the bcc state, and the clouding and subsequent destruction of the initial crystal at the phase transition. Different experimental techniques have been used: optical, 2 NMR, 3,4 neutron scattering, 5 precision barometry, 6 the motion of superconducting wires frozen into solid helium, 1 and an acoustical method.…”

Section: Introductionmentioning

confidence: 99%

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Kinetics of the bcc–hcp transition in He4 off the melting curve

Birchenko

Vekhov

Mikhin

et al. 2006

Low Temperature Physics

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Beryllium (Be) is an important material with wide applications ranging from aerospace components to X-ray equipments. Yet a precise understanding of its phase diagram remains elusive. We have investigated the phase stability of Be using a recently developed hybrid free energy computation method that accounts for anharmonic effects by invoking phonon quasiparticles. We find that the hcp → bcc transition occurs near the melting curve at 0 < P < 11 GPa with a positive Clapeyron slope of 41 ± 4 K/GPa. The bcc phase exists in a narrow temperature range that shrinks with increasing pressure, explaining the difficulty in observing this phase experimentally. This work also demonstrates the validity of this theoretical framework based on phonon quasiparticle to study structural stability and phase transitions in strongly anharmonic materials.Elemental solids usually undergo a series of phase transitions from ambient conditions to extreme conditions [1-3]. Knowledge of their phase diagrams is a prerequisite for establishing their equations of state (EOS), a fundamental relation for determining thermodynamics properties and processes at high pressures and temperatures (PT). However, resolving phase boundaries is challenging for experiments given the uncertainties from several sources, especially at very high PT. Beryllium (Be) is a typical system whose phase diagram remains an open problem despite intense investigations. It assumes a hexagonal close-packed (hcp) structure at relatively low T [1]. Near the melting temperature T M (∼ 1, 550 K at 0 GPa), a competing phase with the body-centered cubic symmetry (bcc) seems to emerge [4-6]. However, not all experiments [7-13] have observed this bcc phase, causing confusion and controversies. Be is important for both fundamental research [14-17] and practical applications. Being a strong and lightweight metal, it has been widely used in a broad range of technological applications in harsh environments and extreme PT conditions, e.g., up to T > 4,000 K and P > 200 GPa [18-22]. The bcc phase of Be was directly observed [4, 5] only at T > 1, 500 K around ambient pressure before melting. Measurements of the temperature dependent resistance suggested that bcc Be is a high pressure phase and the hcp/bcc phase boundary between 0 < P < 6 GPa has a negative Clapeyron slope (−52 ± 8 K/GPa) [6]. However, recent experiments have challenged this conclusion [8, 9, 11-13]. For example, it was reported that bcc Be was not observed for 8 < P < 205 GPa and 300 < T < 4, 000 K [13]. On the theory side, the study of Be's phase diagram using conventional methods encounters significant difficulties. The lattice dynamics of bcc Be is highly anharmonic, and the widely used quasi-harmonic approximation (QHA) and Debye model are not able to capture such effect [23-29]. For this reason, bcc Be and the associated hcp/bcc phase transition remain poorly understood for P < 11 GPa (density < 2.1g/cc) where bcc Be is dynamically unstable at 0 K [24]. At P > 11 GPa, bcc Be is dynamically stabilized by pressure and...

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Section: Introductionsupporting

confidence: 52%

“…This cycle of experimental studies of the kinetics of the bcc-hcp transition in 4 He under different conditions has revealed a number of new features.…”

Section: Discussionmentioning

confidence: 99%

“…In this study we have investigated 4 He crystals with a molar volume of 20.85-21.10 cm 3 / mole, which corresponded to pressures of 25-31 bar temperatures of 1.25-1.90 K.…”

Section: Experimental Techniquementioning

confidence: 99%

Section: A the Bcc Phasementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Kinetics of the bcc–hcp transition in He4 off the melting curve

Birchenko

Vekhov

Mikhin

et al. 2006

Low Temperature Physics

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Fast Diffusion Process in Quenched hcp Dilute Solid 3He–4He Mixture

et al. 2009

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The study of phase structure of dilute 3 He -4 He solid mixture of different quality is performed by spin echo NMR technique. The diffusion coefficient is determined for each coexistent phase. Two diffusion processes are observed in rapidly quenched (non-equilibrium) hcp samples: the first process has a diffusion coefficient corresponding to hcp phase, the second one has huge diffusion coefficient corresponding to liquid phase. That is evidence of liquid-like inclusions formation during fast crystal growing. It is established that these inclusions disappear in equilibrium crystals after careful annealing.

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Kinetic processes at the triple points on the melting curve of He4

Vekhov¹,

Mikhin²,

Polev³

et al. 2005

Low Temperature Physics

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The kinetics of a bcc–hcp structural phase transition on the melting curve of He4 near the triple points (bcc–hcp–He II and bcc–hcp–He I) in the temperature range 1.25–2.0 K is investigated. The capillary blocking method is used to make the samples, and high-precision pressure measurement is used to investigate the kinetics. It is established that far from the triple points exponential functions describe the pressure change accompanying a temperature change on the melting curve. Anomalous behavior of the pressure and temperature of the sample is observed at a transition through the triple points. This attests to remelting of the crystal at a structural phase transition.

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Cited by 4 publications

References 5 publications

Kinetics of the bcc–hcp transition in He4 off the melting curve

Kinetics of the bcc–hcp transition in He4 off the melting curve

Fast Diffusion Process in Quenched hcp Dilute Solid 3He–4He Mixture

Kinetic processes at the triple points on the melting curve of He4

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