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Pressure induced phase transition and amorphization of Na₃ONO₂

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

The high pressure behavior of Na₃ONO₂ has been investigated by Raman spectroscopy and angle-dispersive X-ray diffraction from synchrotron radiation by using the diamond anvil cell technique.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Pressure induced phase transitions and finally amorphization are observed.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

Starting from cubic symmetry at ambient conditions Na₃ONO₂ transforms to a rhombohedral high pressure phase at 0.7 GPa.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Beyond 14.1 GPa, the crystalline structure destabilizes and amorphous state is achieved.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

Introduction

Na₃NO₃, known since 1937, was first regarded as sodium orthonitrite.¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

In 1977, using single crystal X-ray diffraction, the true nature of the compound was discovered to be sodium oxide nitrite, Na₃O(NO₂), with a cubic anti-perovskite type structure.^2–4

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

The NO₂^– ion, disordered at room temperature, occupies the cuboctahedral intersticies formed by the 12 Na⁺ ions of the (Na₃O)⁺ framework.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

Recent neutron scattering studies⁵ have shown that, upon cooling, Na₃ONO₂ undergoes crystallographic phase transitions from a cubic (space group Pm3̄m) to a tetragonal M-Na₃ONO₂ (space group I4/mcm) structure at 240 K, then to a tetragonal T-Na₃ONO₂ (space group P4/mbm) structure at 178 K, and to a tetragonal TT-Na₃ONO₂ (space group P4̄2₁m) structure around 68 K, due to one discontinuous and two continuous ordering processes of the nitrate ion.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

The high pressure behavior of Na₃ONO₂, particularly with regard to the possibility of pressure induced formation of the still unknown NO₃^3– ion, has not been examined.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

This Communication reports the results of X-ray powder diffraction studies of Na₃ONO₂ at high pressures up to 18 GPa at room temperature and 573 K, respectively.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Experimental

Na₃ONO₂ was prepared by the solid state reaction of sodium oxide (Na₂O) and sodium nitrite (NaNO₂) as previously described.⁴

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The crystal structure has been determined at ambient conditions using X-ray and neutron diffraction data.^2–5

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The high pressure experiments were carried out in a diamond anvil cell (DAC) apparatus.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

The sample was loaded in a hole of T301 steel gasket in a glove box and no pressure transmitting medium was used due to the severely hygroscopic character of the sample.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

The pressure was calibrated by the ruby luminescence method.⁶

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

The angle-dispersive X-ray powder diffraction experiments (λ = 0.4154 Å) in a DAC were performed at the beam line ID9 of the European Synchrotron Radiation Facility (ESRF), Grenoble, France.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

Diffraction patterns were recorded on an image plate and then integrated by using the program FIT2D.⁷

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

Raman spectra were collected at room temperature by a Jobin-Yvon LabRAM laser microscope Raman system using a CCD detector with excitation line 632.8 nm (He–Ne laser).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

Results and discussion

The change of Raman spectra of the sample with increasing pressure is shown in Fig. 1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

At ambient condition, two Raman modes, the δ mode and the ν_as mode of the NO₂^– ion,³ are observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

Both Raman modes show a trend to higher frequencies as a function of pressure.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

For the δ mode, the ratio of dω/dP is estimated as 2.13 cm^–1 GPa^–1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

The ν_as mode is located near the Raman peak of diamond, which was used as the high pressure cell anvils, and overlaps with the diamond peak during the compressing process after 10.3 GPa and the dω/dP is estimated as 1.44 cm^–1 GPa^–1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

This enables us to determine the mode Grüneisen parameter , which is evaluated at zero pressure as .

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

The zero pressure bulk modulus, K₀, is estimated as 47.5 GPa for the high pressure phase which is obtained from X-ray diffraction analysis (see later).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

Then γ_G are calculated as 0.125 for the δ mode and 0.052 for the ν_as mode, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

The intensity of the δ mode decreases with increasing pressure, it is observed up to 12.4 GPa, and it completely disappears at 14.1 GPa.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

None of the Raman peaks recover and no Raman active peak, i.e. neither the Raman modes of NO₂^– nor the hypothetical NO₃^3–, which are possible reaction products, could be observed during the decompressing process from 17.4 GPa to 0.2 GPa.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

These results indicate that bonds between N–O in the sample are cleaved above a pressure of 14.1 GPa, and imply that the pressure induced structural conversion is an irreversible process.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

Fig. 2 shows the typical XRD patterns of the sample under various pressures during the compressing processes.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

Using the program GSAS,⁸ the Rietveld refinements were carried out for all of the diffraction patterns.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

The 4th profile function in GSAS was adopted since this function uses the microstrain broadening description⁹ and is more suited to high pressure conditions than other profile functions in GSAS.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

The results reveal that Na₃ONO₂ loses its cubic (space group Pm3̄m) symmetry and transforms to a rhombohedral (space group R3̄m) high pressure phase (R–Na₃ONO₂) at about 0.7 GPa.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

The main structural feature of the anti-perovskite structure remains unchanged during the pressure induced phase transition process due to the similarities of the diffraction patterns to the cubic structure.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Fig. 3 shows a typical refinement result of the powder pattern at 1.3 GPa during compression, in which the background of diffraction pattern is removed first and the refinement converged to R_p = 14.3%, R_wp = 18.7%.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

Fig. 4 shows the lattice parameters of R–Na₃ONO₂ as a function of pressure.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

As a result of the pressure induced distortion the perfect cubic framework of (Na₃O)⁺ transforms to a rhombohedral symmetry, the unit cell angle changes from 90° at ambient conditions to 90.92° at 0.7 GPa, and then gradually to 94.68° at 10.1 GPa.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

When the pressure is increased up to 11.9 GPa, the ratio between intensities of reflections and background in the diffraction pattern decreases.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

After 14.5 GPa the crystalline structure of Na₃ONO₂ is destabilized and an X-ray diffraction amorphous state is achieved.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

The sample remains amorphous when the pressure was increased up to 18 GPa, as it does upon decompressing down to 0.5 GPa.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

This behavior of pressure induced amorphization is in agreement with the high pressure Raman results.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

The reason for the change from crystalline state to an amorphous one seems to be a proceeding decomposition of the sample.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

This decomposition proceeds via cleavage of N–O bonds instead of structural separation into the normal decomposition products Na₂O and NaNO₂ which is shown by the missing reflections in the X-ray diffraction patterns as well as the lack of any NO₂^–ν or δ modes in the Raman spectra of the sample after it was compressed with more than 14.5 GPa.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

As another result from the lack of Raman modes we can exclude the desired formation of the hypothetical NO₃^3– ion.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

The product, possibly a mixture of different phases, does not crystallize again during decompressing and may be inhibited by its topological features because of the lack of activation energy which is necessary for the rearrangement in the solid state.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

The pressure induced amorphization has attracted intensive attention from a fundamental viewpoint over the last two decades.^10–13

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

Amorphous solids, which have no long-range order and are frozen into a particular structural arrangement, are metastable with respect to crystalline phases.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

The different mechanisms of pressure induced amorphization were proposed pertaining to special compounds.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

One mechanism suggests a negative pressure dependence of the melting temperature, thus the extension of the melting curve would pass through the amorphization pressure at room temperature, as observed in ice-I, SiO₂, and Ca(OH)₂ cases of pressure induced amorphization.^10–12

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

For Na₃ONO₂, a slow decomposition instead of melting occurs on heating up to 623 K at normal pressure, this encouraged us to perform high temperature high pressure experiments in order to check the possible negative pressure dependence of the decomposition temperature.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot2

Using an internal heating DAC device, the high pressure angle-dispersive X-ray powder diffraction experiments at a temperature of 573 K were carried out to compare with the room temperature results.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

Firstly we increased the pressure up to 5 GPa, then heated to 573 K and remained at this temperature, then increased the pressure up to 15 GPa.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

The impurity peaks of Na₂O and NaNO₂, which are products of decomposition at ambient condition, are observed together with those of Na₃ONO₂ in the XRD patterns measured under a pressure range of 5–12 GPa.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

Compared to room pressure where the decomposition temperature is 623 K, the relative lower decomposition temperature under higher pressure conditions reveals the characteristic of pressure induced structural destabilization of Na₃ONO₂.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

The X-ray reflections disappear upon compression beyond 13 GPa at 573 K, this pressure is a little lower than the amorphization pressure of 14.5 GPa at room temperature.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

Therefore the negative pressure dependence of decomposition and amorphization temperature was observed.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

In addition to some general interest in the high pressure behavior of Na₃ONO₂, the orientational disorder of nitrite ion inside the sodium oxide framework is of special interest.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot3

The orientational disorder is expected to be reduced under high pressure while the sodium oxide framework transforms from cubic symmetry to the lower rhombohedral symmetric structure, then it has lost its translational symmetry in the amorphous state.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp3

However, the diffraction methods could not really distinguish dynamic disorder phenomena due to the measured patterns being averaged over space and time.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

The relative poor quality of the high pressure diffraction data, due to stress distribution in the high pressure cell without pressure transmitting medium as well as strong preferred orientation phenomena of the sample, hampered us from obtaining clear information on the evolution of orientational disorder of NO₂^– under pressure.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

Therefore the change of the orientational disorder, if present, in the pressure induced decomposition remains unclear.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

Very recently, solid state nuclear magnetic resonance techniques have been used to study the NO₂^– disorder phase transition behavior change upon cooling,¹⁴ and this technique may be applied to disclose the corresponding orientational disorder behavior for the high pressure phase in the future.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

The behavior change of the relative unit cell volume of R–Na₃ONO₂ with pressure is shown in Fig. 5.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

The equation of state of R–Na₃ONO₂ could be fitted into the second order Birch equation of state, in which the pressure derivative of the bulk modulus K₀′ remains as .4¹⁵

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

The zero pressure bulk modulus (K₀) is estimated as 47.5 GPa.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

It indicates that R–Na₃ONO₂ is a soft high pressure phase, which could work well as a pressure transmitting medium itself in DAC during high pressure experiments.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

Conclusion

In summary, Na₃ONO₂ has been investigated by in situ high pressure angle-dispersive X-ray diffraction from synchrotron radiation and Raman spectroscopy.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

A phase transition from cubic symmetry at ambient conditions to a rhombohedral phase at a pressure of around 0.7 GPa is observed.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

Beyond 14.1 GPa, the crystalline structure destabilizes and an amorphous state is achieved.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

The negative pressure dependence of the decomposition temperature of the sample reveals the mechanism of the pressure induced amorphization.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1