1 Synthesis of technetium hydride TcH 1.3 at 27 GPa D. Zhou1 D. V. Semenok1 M. A. Volkov2 I. A. Troyan3 A. Yu. Seregin34 I. V. Chepkasov1 D. A.

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Synthesis of technetium hydride TcH1.3 at 27 GPa

D. Zhou,1,* D. V. Semenok,1,* M. A. Volkov,2 I. A. Troyan,3 A. Yu. Seregin,3,4 I. V. Chepkasov,1 D. A.

Sannikov,5 P. G. Lagoudakis,5 A. R. Oganov,1 and K. E. German2,*

1 Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld. 1, Moscow 121205, Russia

2 A. N. Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences, Department of

Radiochemistry, 31 Leninsky Prospekt, Moscow 119991, Russia

3 Shubnikov Institute of Crystallography, Federal Scientific Research Center Crystallography and Photonics, Russian Academy of

Sciences, 59 Leninsky Prospekt, Moscow 119333, Russia

4 National Research Center “Kurchatov Institute”, Ploshchad' Akademika Kurchatova, 1, Moscow 123182, Russia

5 Center for Photonics and Quantum Materials, Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld. 1,

Moscow 121205, Russia

*Corresponding authors: d.zhou@skoltech.ru, dmitrii.semenok@skoltech.ru, guerman_k@mail.ru

Abstract

In this work, we synthesize and investigate lower technetium hydrides at pressures up to 45 GPa using

the synchrotron X-ray diffraction, reflectance spectroscopy, and ab initio calculations. In the Tc–H system, the

hydrogen content in TcHx phases increases when the pressure rises, and at 27 GPa we found a new hexagonal

(hcp) nonstoichiometric hydride TcH1.3. The formation of technetium hydrides is also confirmed by the

emergence of a new reflective band at 450–600 nm in the reflectance spectra of TcHx samples synthesized at

45 GPa. On the basis of the theoretical analysis, we proposed crystal structures for the phases TcH0.45±0.05

(TcH16H7) and TcH0.75±0.05 (Tc4H3) previously obtained at 1–2 GPa. The calculations of the electron–phonon

interaction show that technetium hydrides TcH1+x do not possess superconducting properties due to the low

electron–phonon interaction parameter (λ ~ 0.23).

Keywords: technetium, hydrides, high pressures, reflectance spectroscopy, diamond anvil cells

Introduction

The science and technology of polyhydride materials currently have two main vectors of development:

materials for hydrogen storage and high-temperature superconductors. The first direction1 recently received a

significant boost with the discovery of pressure-stabilized polyhydrides with ultrahigh hydrogen content (up

to 63 wt %) such as the molecular complexes of methane (CH4)3(H2)25,2 hydrogen iodide (HI)(H2)13,3 strontium

and barium polyhydrides SrH224 and BaH12,5 and various rubidium and cesium polyhydrides.6 Although the

stabilization of most polyhydrides currently requires considerable pressures of at least several gigapascals

(Table 1), the conditions for their stabilization are continuously improving, which gives hope for a discovery

of new stable hydrogen storage compounds with record capacity in the future.

Table 1. Hydrogen capacity of polyhydrides.

Compound Stabilization

pressure, GPa

Hydrogen

content, wt %

CH4 0 25

NH3BH3 0 19

RbH~9 8 9.5

CsH~17 10 11

BaH12 75 8

2

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SrH22 80 20

Xe(H2)8 5–8 11

(CH4)3(H2)25 = СH20.7 10 63

(HI)(H2)13 9 17

The second promising area of research in hydride chemistry is the designing of high-temperature

superconductors with a near-room temperature of superconductivity.7 At the moment, a well reproducible

record superconductor is fcc-LaH10 with a critical superconductivity temperature TC of 250 ± 5 K.8–9

Superconductivity is distributed unevenly among polyhydrides, being a distinctive feature of mostly cubic and

hexagonal hydrides such as XH6, XH9, XH10, and tetragonal XH4, with the atomic hydrogen sublattices

stabilized at 100–200 GPa.10–11 In this pressure range, the critical temperatures of the best superhydrides are in

the range TC [K] ⸦ (P, 2P) [GPa]. The hydride-forming element can be sulfur, an alkaline earth metal (Mg,

Ca, Sr, Ba), or have only 1–2 d or f electrons (La, Y, Zr, Th, Ce, Lu, etc.). For all other elements of the periodic

table (the “empty” zone), polyhydrides are either not formed or do not exhibit superconducting properties.

In this research, we investigate the Tc–H system, belonging to this “empty” zone, at pressures up to

45 GPa to verify theoretical predictions on the distribution of superconducting properties in polyhydrides.12

Despite the obvious interest in hydrogen-saturated compounds like K2TcH9,12 only a few papers on

thermodynamic and quantum mechanical calculations of the Tc–H system for gaseous states of hydrides are

found in the literature before 2021.13–16 For example, the calculations of the enthalpy of formation of

technetium monohydride17 showed that ΔfH°m(½TcH) = +9 kJ/mole, that is, TcH must decompose

spontaneously at ambient pressure and can be stabilized only when the pressure is increased.

Experimental works in the synthesis of technetium hydrides at elevated pressures have shown that

metallic technetium has a little tendency to react with H2 under the ambient temperature and pressure

conditions.18 Spitsyn et al.18 reported the synthesis of TcH0.73±0.05 via a direct reaction of Tc with hydrogen gas

at pressures up to 1.9 GPa and a temperature of 573 K. The resulting hydride formed a single phase with a

hexagonal lattice: a = 2.805 ± 0.02 Å, c = 4.455 ± 0.02 Å. At an elevated pressure (2.2 GPa), the same team

obtained two hexagonal phases with the compositions TcH0.5 (ε1) and TcH0.78 (ε2).19 The ε1  ε2 phase

transition, observed at about 1 GPa, was confirmed by resistivity measurements.

The formation of nonstoichiometric hydrides TcH0.45, TcH0.69,20–21 and TcHm, where m = 0.26,22 0.385,

0.485, and 0.765,23 has been proved using the neutron powder diffraction. The hydrides with m = 0.45 and 0.69

have been indexed in the hexagonal space group hcp: a = 2.801 ± 0.004 Å, с = 4.454 ± 0.01 Å for TcH0.45;

a = 2.838 ± 0.004 Å, с = 4.465 ± 0.01 Å for TcH0.69. Three unexpected peaks in the neutron diffraction patterns

have been observed for TcH0.45 and interpreted as the evidence of a superstructure. The authors concluded that

hydrogen atoms occupy octahedral spaces in the technetium lattice. 20–23

The studies of the transport properties and superconductivity of the obtained lower hydrides showed that

hydrogen is highly negative to the electron–phonon interaction in technetium. The critical temperature TC of

the pure metal is about 7.73 K at normal pressure, which is one of the highest values among the pure elements.24

However, TcH0.73±0.05 (ε2) does not have a transition to the superconducting state above 2 K, probably because

of the increasing Tc–Tc distance.18,20,25–26

Theoretical studies of stable compounds in the Tc–H system have been previously conducted using the

density functional theory (DFT) methods at high and ultrahigh pressures up to 300 GPa.27 It has been found

that increasing the pressure leads to the formation of new polyhydrides: P63/mmc-TcH (stable in the range of

0–

200 GPa), I4/mmm-TcH2 (stable above 64 GPa), Pnma-TcH3 (stable above 79 GPa), and P42/mmc-TcH3

(forms at about 300 GPa), in which the charge is partially transferred from the technetium atoms to hydrogen.

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The theoretically calculated superconducting properties of technetium hydrides are weakly expressed, with T

not exceeding 11 K.

In this work, we synthesized and characterized technetium hydrides TcH

1+x

at pressures up to 27 GPa

using the X-ray powder diffraction and at pressures up to 45 GPa using the reflection spectroscopy in diamond

anvil cells (DACs).

Results and Discussion

Structure search

At present, theoretical analysis and evolutionary structural search are among the cornerstones in hydride

chemistry

28–32

because the exact positions of hydrogen atoms in polyhydrides at high pressures cannot be

established using experimental methods. We performed an evolutionary crystal structure search for

thermodynamically stable phases at 5, 25, and 50 GPa in the Tc–H system using the USPEX code.

33–35



In terms

of thermodynamics, the Tc–H system belongs to those where metal-rich phases dominate hydrogen-rich phases

(e.g., Ba–H,

Sr–H,

etc.).

Figure 1. Calculated convex hulls for the Tc–H system at (a) 50 GPa, (b) 5 GPa, and (c, d) the most stable crystal

structures of pseudohexagonal TcH

(x = 0.75 ± 0.12). Inset: (pseudo)hexagonal structures of TcH and Tc

, where

hydrogen (H) occupies octahedral voids. The predicted XRD patterns of these structures and their CIF files are given in

the Supporting Information.

The calculations resulted in a phase diagram (convex hull, Figure 1) of technetium hydrides which shows

that at 25 (see the Supporting Information, Figure S1) and 50 GPa the highest hydrogen content is achieved in

/mmc-TcH, whereas all higher hydrides are thermodynamically unstable. Many nonstoichiometric TcH

hydrides (x < 1) lie close to the convex hull between Tc and TcH, which speaks in favor of the possible

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formation of hydrides with intermediate compositions (0 < x < 1) at low pressures or hydrogen deficiency. The

most stable phases at 50 GPa are pseudohexagonal P1-Tc

, P1-Tc

, and P1-Tc

(Figure 1). At 5 GPa,

the thermodynamically stable phases are Tc

and Tc

, which are candidates for previously experimentally

found compounds TcH

0.75±0.05

(ε

) and TcH

0.5±0.05

(ε

), respectively.

Experimental synthesis

Technetium samples mixed with ammonia borane NH

were heated in DACs T1 and T2 by a pulsed

laser for several hundred microseconds to 1500 K at pressures of 34 GPa (DAC T1, 300 μm culet) and 45 GPa

(DAC T2, determined via the Raman signal of diamond

). After the laser heating, the pressure in DAC T1

dropped to 27 GPa. Low-pressure X-ray diffraction studies were carried out on a synchrotron source of the

Kurchatov Institute (KISI-Kurchatov), station RKFM (λ = 0.62 Å, 20 keV, beam width was about 50 μm).

Figure 2. X-ray powder analysis of technetium hydrides in DAC T1. Microphotographs of the loaded Tc sample (a)

before and (b) after the laser heating, where the pressure significantly decreased. (c) Experimental XRD pattern measured

at 300 K and the Le Bail refinement of the unit cell parameters of P6

/mmc-TcH at 12 GPa. The experimental data, fit,

and residue are shown in red, blue, and green, respectively. The unidentified reflections are indicated by asterisks. Inset:

pressure dependence of the c/a ratio. (d) Experimental diffraction pattern of TcH.

Figure 2a,b shows a substantial change in the character of the technetium surface after the laser heating.

As we will see below, the spectral reflectivity of the sample also changes significantly. The XRD analysis

shows the presence of one hexagonal phase (Figure 2c), whose cell volume at pressures above 10 GPa slightly

exceeds the values theoretically calculated for P6

/mmc-TcH (Table 2), corresponding to compositions

TcH

1.1–

1.3

. Some XRD patterns also show an hcp admixture of Re (or Tc), which is due to the large width of

the X-ray beam (~50 μm). As the pressure in DAC T1 decreases, so does the hydrogen content in the

5

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compound, and at 1–2 GPa the cell parameters of TcHx approach the literature data for the composition TcH0.73

(Figure 3).18–19,23 This allows us to conclude that stoichiometric TcH is thermodynamically unstable below

10 GPa and loses hydrogen already at 300 K.

Table 2. Experimental unit cell parameters of synthesized technetium hydridesTcH1+x and metallic

technetium (VTc).37

Pressure, GPa a, Å c, Å c/a V, Å3 (Z = 2) V, Å3/Tc VTc, Å3/Tc

27 2.845 4.472 1.572 31.35 15.67 13.28

20 2.849 4.479 1.572 31.49 15.74 13.51

12 2.864 4.492 1.568 31.91 15.95 13.79

7 2.856 4.488 1.571 31.70 15.85 14.00

~1 2.838 4.475 1.572 31.22 15.61 14.30

~2, (TcH0.73)

~2,

(

TcH0.45

)

2.838

2.801

4.465

4.454

1.573

1.590

31.14

30.62

15.57

15.31

14.21

The reaction of technetium with hydrogen at high pressures is very similar to the behavior of the Mo–H

system, where, also at a pressure of about 15–20 GPa, a nonstoichiometric phase MoH1.35 forms.38 However,

for example, the behavior of the Re–H system is different: at 20 GPa, only ReH0.38, a hydride with substantially

lower hydrogen content, is formed.39 An estimation of the saturation composition of technetium hydride TcH1+x

at 27 GPa is possible from volumetric considerations. Because the volume expansion per Tc atom is measured

using X-ray diffraction, the composition H/Tc of the saturated Tc hydride can be estimated by comparing this

volume expansion with the expected volume expansion per H atom. In the hcp structure, the occupation of all

octahedral voids by hydrogen leads to TcH stoichiometry. When more hydrogen is used, it is only possible to

place all extra hydrogens in the tetrahedral voids. Each hydrogen atom placed in an octahedral interstitial site

in a close-packed lattice of technetium expands the lattice by about 1.86 Å3 per H atom, whereas in a tetrahedral

site the volume expansion is 2.2–3.2 Å3 per H atom.40 Considering that ΔV = V(TcH1+x) – V(Tc) = 2.39

(27 GPa), 2.23 (20 GPa), 2.16 (12 GPa), and 1.85 Å3/Tc (7 GPa), we can estimate the maximum hydrogen

content of the obtained hydride as TcH1.3 (Figure 3a), which is quite close to the results of the first-principles

calculations.

Hydrogen has a negative effect on the superconducting properties of technetium despite the fact that it

only slightly directly affects the electronic structure of Tc hydrides, contributing almost nothing to the electron

density of states at the Fermi level (Figure 4d, Supporting Information, Figures S6-7). This is due to the fact

that hydrogen occupies the octahedral voids in the hexagonal lattice of technetium, increasing the parameters

of the unit cell, which is equivalent to introducing a formally negative pressure to hcp-Tc. This concept is

schematically shown in Figure 3d. For instance, an extrapolation of the equation of state of Tc (Figure 3a,

Supporting Information Figure S3) to the negative pressure region shows that the cell volume of the

synthesized hydride TcH1+x corresponds to a pressure of about –30 GPa. At the same time, the region of

negative pressures from 0 to –30 GPa, where the unit cell of Tc is only slightly expanded, is unexplored, and

a local increase in the critical temperature TC expected there requires either a compression of TcH above

50 GPa, or controlled hydrogenation with a very small amount of H2. From a practical point of view, small

levels of hydrogenation with a controlled expansion of the unit cell are achievable using the electrochemical

approach.41 At the same time, as the pressure increases from 0 to 1.5 GPa, the critical temperature of technetium

decreases with a slope of dTC/dP = –0.125 K/GPa.42

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1 SynthesisoftechnetiumhydrideTcH1.3at27GPaD.Zhou,1,*D.V.Semenok,1,*M.A.Volkov,2I.A.Troyan,3A.Yu.Seregin,3,4I.V.Chepkasov,1D.A.Sannikov,5P.G.Lagoudakis,5A.R.Oganov,1andK.E.German2,*1SkolkovoInstituteofScienceandTechnology,BolshoyBoulevard30,bld.1,Moscow121205,Russia2A.N.FrumkinInstituteofPhysicalCh...

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1 Synthesis of technetium hydride TcH 1.3 at 27 GPa D. Zhou1 D. V. Semenok1 M. A. Volkov2 I. A. Troyan3 A. Yu. Seregin34 I. V. Chepkasov1 D. A.

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