Kinetically Controlled Low-Temperature Solid-State Metathesis of Manganese Nitride Mn3N2

Article abstract of DOI:10.1021/acs.chemmater.9b01565

The synthesis of inorganic metal nitrides poses a challenge due to the low reactivity of N₂ gas at low temperatures, yet entropy driven formation of N₂ gas at high temperatures. In contrast, synthetic approaches using more activated forms of nitrogen can be used to overcome the inertness of N₂, but increased exothermicity can also result in diminished stoichiometric control and the activation of deleterious competing pathways. Here, kinetically controlled solid-state metathesis reactions are used to prepare Mn₃N₂ without the use of experimental conditions that increase the chemical potential of nitrogen and are known to produce phase impurity (e.g., NH₃, N₂-based plasma, azides, or high pressure). The solid-state metathesis reaction between MnCl₂ and Mg₂NCl or Mg₃N₂ is shown to generate Mn₃N₂, a phase on the border of stability. Highly exothermic control reactions performed with Li₃N, Ca₃N₂, and Ca₂NCl yield poorly crystalline, nitrogen-deficient Mn-N phases and N₂ gas. The reactions with Mg₂NCl and Mg₃N₂ do not self-propagate and have the lowest predicted free energies of reaction. A series of reactions performed at different times and temperatures, as well as in situ synchrotron X-ray diffraction, illustrate the importance of kinetic competence, and the results implicate the mechanism for this competence: the formation of a solid-solution, Mg_xMn_1-xCl₂, between the halide precursor (MnCl₂) and the halide product (MgCl₂) coupled to a mildly exothermic reaction. Kinetically controlled solid-state metathesis continues to provide an avenue toward the synthesis of materials that cannot be prepared under traditional, high-temperature ceramic methods.

Full text of DOI:10.1021/acs.chemmater.9b01565

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Article
Kinetically-controlled low-temperature solid-
3
2
state metathesis of manganese nitride MnN
Erik G. Rognerud, Christopher L. Rom, Paul K. Todd, Nicholas Ryan
Singstock, Christopher J. Bartel, Aaron M. Holder, and James R. Neilson
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01565 • Publication Date (Web): 20 Aug 2019
Downloaded from pubs.acs.org on August 24, 2019
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Kinetically-controlled low-temperature solid-state metathesis of manganese nitride Mn N
3
2
a
a
a
b
Erik G. Rognerud, Christopher L. Rom, Paul K. Todd, Nicholas Ryan Singstock, Christopher
b
b,c
a*
J. Bartel, Aaron M. Holder, James R. Neilson
Department of Chemistry, Colorado State University; Fort Collins, CO USA 80523-1872
a
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Department of Chemical and Biological Engineering, University of Colorado; Boulder, CO
80309, USA
c
National Renewable Energy Laboratory; Golden, CO 80401, USA
*
james.neilson@colostate.edu
ABSTRACT
The synthesis of inorganic metal nitrides poses a challenge due to the low reactivity of N2
gas at low temperatures, yet entropy driven formation of N gas at high temperature. In contrast,
2
synthetic approaches using more activated forms of nitrogen can be used to overcome the
inertness of N , but increased exothermicity can also result in diminished stoichiometric control
2
and the activation of deleterious competing pathways. Here, kinetically-controlled solid-state
metathesis reactions are used to prepare Mn N without the use of experimental conditions that
3
2
increase the chemical potential of nitrogen and are known to produce phase impurity (e.g., NH3,
N -based plasma, azides, or high pressure). The solid-state metathesis reaction between MnCl
2
2
and Mg NCl or Mg N is shown to generate Mn N , a phase on the border of stability. Highly
2
3
2
3
2
exothermic control reactions performed with Li N, Ca N , and Ca NCl yield poorly crystalline,
3
3
2
2
nitrogen-deficient Mn-N phases and N formation. The reactions with Mg NCl and Mg N do
2
2
3
2
not self-propagate and have the lowest predicted free energies of reaction. A series of reactions
performed at different times and temperatures, as well as in situ synchrotron X-ray diffraction,
illustrate the importance of kinetic competence and implicates the mechanism for this
competence: the formation of a solid-solution, Mg Mn Cl , between the halide precursor
x
1-x
2
(
MnCl ) and the halide product (MgCl ) coupled to a mildly exothermic reaction. Kinetically-
2
2
controlled solid-state metathesis continues to provide an avenue toward the synthesis of materials
that cannot be prepared under traditional, high-temperature ceramic methods.
INTRODUCTION
Transition metal nitrides have developed considerable interest in fields such as ceramics,
photovoltaics, and batteries because of their unique bonding patterns, high hardness, photoactive
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1,2
band-gap and many more properties. Unfortunately, the development of these materials is
limited by their difficult synthesis. The formation of nitrogen gas is often a kinetic (at low
temperatures) and entropic (at high temperatures) competitor to the formation of metal nitrides.3
•
–
Activated nitrogen, as found with nitrogen plasmas (N ) or unstable molecules (N ) has recently
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garnered success in the synthesis of new binary and ternary nitrides. However, the
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interdiffusion of nitride-containing precursors, as in solid-state metathesis (SSM), offers new
opportunities to synthesize metal nitrides. SSM reactions provide a low activation temperature
alternative to the solid-state synthesis of many materials, which includes nitrides.6–8 The SSM of
metal nitrides has typically involved the combination of transition metal halides with alkali or
alkaline earth nitrides. Many of these reactions are sufficiently exothermic as to be self-
propagating (cf., 1300 °C for ZrN); however, upon self-heating, kinetic control may be lost and
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N (g) may be formed instead.
2
Solid-state metathesis reactions need not be self-propagating due to the exothermic self-
heating of the reaction. Our group has shown that solid-state metathesis reactions can be
selected to enable kinetic control, as metastable products can be isolated from low-temperature,
non-propagating reactions.10–13 Furthermore, we recently recognized the role of kinetic
competence in solid-state reactions, stipulating that the precursors have the necessary ability to
perform the reaction at the given temperature, as it pertains to atomic transport and reactivity;
this approach enables the synthesis of materials that are unstable at higher temperatures (e.g.,
1
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orthorhombic YMnO ) .
3
+δ
Manganese nitrides offer a platform for investigating controlled, stoichiometric nitride
synthesis. At least four manganese nitride phases are known and include Mn N, Mn N, MnN
4
2
15,16
(
i.e., Mn N ), and Mn N .
Of particular interest is the non-interstitial compound, Mn N ,
3 2
6
5+x
3
2
1
7
which has been sought after for catalysis and high-temperature antiferromagnetism (T = 927
N
K).16 While Mn N can be prepared from reaction of Mn and N (g), and Mn N from reaction of
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Mn and NH (g), compounds with increased nitrogen composition, such as Mn N and Mn N ,
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have required an increased chemical potential of nitrogen, as enabled by plasma-enhanced
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16
sputtering or molecular beam epitaxy, high pressure (N or NH ), reaction with NaN , and
2
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3–25
via LiNH or NaNH salt metathesis or flux.
In many of the reported syntheses for Mn N ,
3 2
2
2
decomposition products include Mn N or Mn N (with the necessary production of nitrogen
4
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gas).25 This contrasts the expected more thermodynamically favorable formation of “MnN”
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,15
(Mn N ) as a decomposition product based on the Mn-N phase diagram.
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This contribution describes how Mn N is produced using kinetically-competent Mg-
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based reactions, 2 Mg NCl + 3 MnCl → Mn N + 4 MgCl and Mg N + 3 MnCl → Mn N +
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MgCl . Synthesis at higher temperatures loses stoichiometric control, as Mn N (and
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presumably N (g)) forms. From reactions quenched at lower temperatures, as well as in situ X-
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ray diffraction, solid-solution behavior for Mg Mn Cl is observed, suggesting that this
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intermediate plays a role in enabling kinetic competence of the solid-state reaction. Control
reactions with Li N, Ca N , and Ca NCl yield poorly crystalline, nitrogen-deficient Mn-N phases
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and N formation; these control reactions are nearly twice as enthalpically favorable, and the
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halide salt by-products are known to not form solid-solutions with MnCl . By “turning down the
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heat,”26 the solid-state metathesis reactions of 2 Mg NCl + 3 MnCl and Mg N + 3 MnCl yield
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stoichiometrically-controlled reactions and formation of Mn N .
3
2
Figure 1: Schematic representation of the solid-state reaction pathway through the solid solution
Mg Mn Cl ) intermediate that plays a key role in enabling kinetic competence. Shown is the
(
x
1-x
2
transformation of Mg NCl (top left) or Mg N (bottom left) into Mn N (right).
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2
METHODS
All precursors were prepared and stored in an argon filled glove-box (O < 0.1 ppm, H O
2
2
<
0.5 ppm). Mg NCl was synthesized as previously reported with Mg N (Alfa Aesar: 99%
2
3
2
2
7
purity) and MgCl (Sigma Aldrich: 98% purity). Stoichiometric amounts of Mg N and MgCl
2
2
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were combined in an agate mortar and pestle and ground into a homogenous tan powder. The
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powder was then pressed (P ~1 ton) into a dense pellet and placed in a quartz ampoule. This
ampoule was then brought out of the glovebox and quickly sealed under vacuum (P < 10 mTorr)
using an oxygen/methane torch. The sealed ampoule was then heated at 5 °C/min in a muffle
furnace to a set point of 550 °C. The reaction was held at temperature for 7 days, and then
allowed to cool to room temperature. The reactions reported here are sensitive to the precise
temperature such that thermal gradients within the muffle furnaces influence reproducibility.
Pelletized samples experienced temperatures approximately 20 to 40 °C below the temperature
reported by the muffle furnace thermocouple, as probed by an external thermocouple. All
temperatures reported are the nominal temperature set point, with estimates to the actual
uncertainty described in the text.
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The reactions of MnCl (Alfa Aesar: 97% purity) with Mg N or Mg NCl to produce
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Mn N were performed under a similar procedure. Stoichiometric amounts of reactants were
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combined with MnCl in an agate mortar and pestle resulting in a homogenous pink powder. This
2
powder was then pressed into a pellet (P ~1 ton) and placed in a quartz glass ampoule within an
inert glovebox. Out of the glovebox, the ampoule was quickly sealed under vacuum (P < 10
mTorr) with an oxygen/methane torch. Sealed ampoules were heated in a conventional muffle
furnace for various heating schedules described in the text; all reactions were performed with a
ramp rate of 1 °C/min and furnace cool.
Control reactions were run with Li N (Alfa Aesar: 99.4% purity), Ca N (Alfa Aesar:
3
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2
9
9% purity), and Ca NCl. Ca NCl was synthesized from CaCl and Ca N as previously
2 2 2 3 2
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reported. The control reactions were performed under the same conditions as discussed above.
The products of all reactions were characterized by powder X-ray diffraction (PXRD)
and Rietveld analysis. PXRD was performed using a Bruker DaVinci diffractometer with CuKα
X-ray radiation. All samples were prepared for PXRD from within the glove-box by placing
powder on silicon wafers to reduce the background and covered in Kapton® tape to prevent
exposure to the atmosphere. Some PXRD samples were prepared for quantitative Rietveld
analyses by grinding known masses of silicon powder (as an internal standard) with known
masses of the sample. Rietveld analyses were performed using the software TOPAS v6 (Bruker
AXS) to obtain values for cell parameters, occupancies, and phase fractions.
In situ PXRD studies were conducted at 17-BM-B of the Advanced Photon Source at
Argonne National Lab (흀 = 0.24125 Å). Precursors were packed in extruded borosilicate
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capillaries and flame sealed under vacuum (<10 mTorr). The tip of a thermocouple was placed
against the capillary, within 3 mm of the X-ray beam, and two parallel resistive heaters
controlled the sample temperature during the measurements with much greater accuracy than the
muffle furnaces. Data collection was carried out with a 2048 × 2048 PerkinElmer 2D plate
detector at a distance of 70 cm from the sample. The 2D diffraction patterns were then radially
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integrated with GSAS-II. Sequential Rietveld refinements were conducted using TOPAS v6
Bruker AXS) to identify phase fraction, the occupancy (x) of Mg Mn Cl and lattice
(
x
1–x
2
parameters. Strain was also refined. The Mn N phase was only refined for the dwell scans to
2
prevent poor convergence of the calculation, although small peaks can be qualitatively observed
for data collected above T = 385 °C.
The thermochemistry of manganese nitride stability and metathesis reaction energetics
were computationally evaluated at finite temperatures using quantum chemical calculations. All
atomic structure and total energy calculations were performed using the projector augmented-
wave method within the VASP code,30,31 and in accordance with our previous calculation
2
parameters used for computing binary nitride phase diagrams and evaluating stability. In brief,
the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation exchange-correlation
3
2
functional was employed for all calculations, with a plane-wave energy cutoff of 520ꢀeV, and
Γ-centered Monkhorst-Pack k-point grid. The energy cutoff, k-point density, and related
convergence settings were sufficient to achieve total energy convergence of < 0.5ꢀmeV/atom for
2
all calculations. These computational parameters were selected to enable the direct evaluation of
thermodynamic stability and reactions energetics involving all potential competing compounds
and formation enthalpies computed within this work and those reported within the Materials
33
Project database. We note that our predicted Mn-N convex hull phase diagram is consistent
with the results reported using the meta-GGA SCAN functional and elemental reference energy
2
correction schemes for evaluating thermochemistry to well within the error bars of these
3
4
methods, yet there is a discrepancy between these methods in whether Mn N is a near hull or
3
2
on the hull compound (metastable or stable). Therefore, we propagate the mean absolute error
(MAE) of ~35 meV/atom for evaluating the 0K decomposition enthalpy and near hull stability of
3
4
Mn N throughout the computational predictions provided in this work. This MAE is
3
2
applicable towards the evaluation of solid phase reaction energetics, such as the decomposition
of compounds into other compounds, etc., and is of comparable magnitude to the experimental
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uncertainty (~ 22 meV/atom for a binary compound) in evaluating these enthalpies.34 Compound
free energies at finite temperatures were computed using a statistical-descriptor for inorganic
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crystalline solid Gibbs formation energies. Crystal Orbital Hamilton Populations (COHP) used
in analyzing the bonding and electronic structure were obtained using the LOBSTER code.36
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RESULTS
The structure of manganese nitride, Mn N , adopts a body-centered tetragonal unit cell
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and is pictured in Figure 1. Nitrogen atoms occupy two-thirds of the octahedral sites allowing
for two distinct manganese atom locations. One manganese atom is linearly coordinated to the
interstitial nitrogens, while the other atom is coordinated in a square pyramidal fashion to five
nitrogen atoms. The unit cell parameters of synthesized Mn N are within uncertainty (evaluated
3
2
from PXRD) of those found in the literature; a = 2.974 Å and c = 12.126 Å (I4/mmm).16
Figure 2: PXRD data of the products from 2 Mg NCl + 3 MnCl at different muffle furnace set
2
2
points (12 h dwell times, actual reaction temperatures ~20-40 °C lower). The data and fitted
curves are shown with a constant offset for clarity. Data are shown as black dots; difference
between the data and calculated in blue lines. Contributions from each phase (determined via
Rietveld method) are displayed as colored lines except for the Mn N compound, which is shown
3
2
as a solid red fill. The data show a decrease in diffraction intensity from the reactants with
increasing temperature, concomitant with an increase in Mn N and MgCl phases. At higher set
3
2
2
points (450 °C to 500 °C), Mn N is observed while Mn N decreases in intensity.
2
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Stoichiometric control of the reaction of 2 Mg NCl + 3 MnCl ⟶ Mn N + 4MgCl is
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optimized at muffle furnace set points of 400 °C. PXRD patterns of the reaction products over a
temperature set point range from 300 °C to 500 °C are shown in Figure 2, and the respective
quantitative phase analysis is tabulated in Table SI. At lower temperatures, solid-solution
Mg Mn Cl phases are also observed. Set points above 400 °C lead to the decomposition of
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Mn N to the nitrogen deficient phase, Mn N. Similar trends are noted for the reaction Mg N +
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MnCl → Mn N + 3 MgCl (Figure S1).
2 3 2 2
Figure 3. False color image of in situ PXRD data of 2Mg NCl + 3MnCl ramped from room
2
2,
temperature to 400 °C at a rate of 10 °C/min, showing a focused Q range. The horizontal
dashed line denotes the time at which 400 °C is reached and maintained. We estimate the error
in temperature at the sample is ±1 °C. Tick marks show peak positions for phases in the final
scan shown at top. The full Q range is shown in Figure S2.
In situ PXRD patterns reveal direct conversion of Mg NCl and MnCl to MgCl and
2
2
2
Mn N , with a trace quantity of Mn N detected at higher temperatures after isothermal dwell
3
2
2
(
Figure 3). No other crystalline intermediates are detected. In addition to the observation of
thermal expansion during heating, the peaks from Mg NCl decrease in intensity over time, while
2
Mn N forms after 35 minutes (T = 370 °C). As the reaction proceeds, the (2 1¯ 0) peak of
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Mg Mn Cl decreases in Q (due to contraction along the a axis) and decreases in intensity,
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while the (10 8¯ ) peak grows in intensity, suggesting Mg substitution at Mn sites. Mn N formation
2
is observable at 400 °C (38 min). These data reveal a narrow temperature window in which
Mg NCl is a kinetically-competent precursor for formation of Mn N , but above which
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formation of Mn N competes and Mn N decomposes (Figure S4). Within this window
2
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2
however, Mn N formation is not detectable by ex situ PXRD, even at reaction dwell times as
2
long as 1 month (Figure S3).
Figure 4. Summary of Rietveld analysis results from the in situ PXRD experiment of
2Mg NCl + 3MnCl ramped from room temperature to 400 °C at a rate of 10 °C/min (Figure
2
2,
3
). (a) The phase fraction of Mg NCl decreases as manganese nitride phases form, which
2
corresponds with (b) a change in occupancy and (c) a contraction in the a lattice parameter of
Mg Mn Cl . (d) The c lattice parameter of Mg Mn Cl expands slightly.
x
1-x
2
x
1-x
2
Solid-solution behavior of the MnCl precursor with the MgCl product is observed
2
2
through Rietveld analysis of the in situ PXRD data (Figure 4). The increase in Mg occupancy
during the dwell (Figure 4b) mirrors the decrease in the a lattice parameter for the Mg Mn Cl
2
x
1–x
phase (Figure 4c), as the phase shifts from predominantly MnCl (a
= 3.711 Å) to MgCl2
298K
2
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(
a298K = 3.636 Å). These data are consistent with ex situ PXRD data under various reaction
conditions (Figures S5 and S6). However, the in situ data illustrate that the rate of increasing
phase fraction of Mn N is more gradual in time than are the changes to Mg Mn Cl .
3
2
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Figure 5: (a) PXRD of products from the reaction of 2 Li N + 3 MnCl after self-propagation
3
2
during mild heating from flame sealing the reaction ampoule. The reaction products were
exposed to air, resulting in deliquescence of the LiCl to reveal only crystalline Mn N and Mn N.
2
4
(b) Products from Ca N + 3 MnCl at 400 °C (muffle furnace set point) for 12 hours detectable
3
2
2
by PXRD. The reaction produced CaCl (68.58 mol%), as well as the nitrogen deficient
2
manganese nitrides; Mn N (11.51 mol%) and Mn N (19.90 mol%). This suggests loss of
4
2
nitrogen in the form of N (g). (c) Products from 2 Ca NCl + 3 MnCl at 400 °C (muffle furnace
2
2
2
set point) for 12 hours detectable by PXRD. Three manganese nitride compounds were formed
including Mn N (6.76 mol%), Mn N (19.00 mol%), and Mn N (2.75 mol%) along with CaCl
4
2
3
2
2
(71.49 mol%).
Several control reactions were attempted with other nitrogen-containing precursors in order to
understand the thermochemical factors responsible for Mn N formation. The reaction, 2 Li N +
3
2
3
3
MnCl → Mn N + 6 LiCl, instead produces nitrogen deficient manganese nitrides: Mn N and
2 3 2 4
Mn N. The reaction mixture self-propagated under mild heat (CAUTION! e.g., while flame
2
sealing; Figure 5a). The reaction, Ca N + 3 MnCl → Mn N + 3 CaCl also yields nitrogen-
3
2
2
3
2
2
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deficient Mn-N phases, Mn N (11.51 mol%) and Mn N (19.90 mol%), as well as CaCl (68.58
4
2
2
mol%; Figure 5b). The reaction, 2 Ca NCl + 3 MnCl → Mn N + 4 CaCl , produces a mixture
2
2
3
2
2
of the nitrides Mn N (2.75 mol%), Mn N (6.76 mol%), and Mn N (19.00 mol%), as well as
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CaCl (71.49 mol%; Figure 5c).
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DISCUSSION
The reactions, 2 Mg NCl + 3 MnCl → Mn N + 4 MgCl and Mg N + 3 MnCl →
2
2
3
2
2
3
2
2
Mn N + 3 MgCl proceed stoichiometrically to yield crystalline Mn N . These synthesis
3
2
2
3
2
reactions contrast previous efforts, which rely on conditions aimed at increasing the chemical
potential of nitrogen, often through pressure or highly reactive precursors.16,20–25 The reaction
here instead proceeds at relatively low temperature (~370 °C) via the interdiffusion of
magnesium-based nitride precursors.
Figure 6: Convex hull phase diagram of binary manganese nitride compounds at 0 K, 300 K,
6
00 K, and 900 K. ΔG (T) is the Gibbs formation energy at the specified temperature, x is the
f
fraction of nitrogen in the compound, and the shaded regions indicate the energetic resolution
of the computational approach in evaluating the hull stability. The predicted formation energy
of Mn N relative to Mn N and MnN places it near the border of stability and being metastable
3
2
2
towards decomposition into these compounds, which is consistent with the experimentally
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observed challenge of readily synthesizing Mn N using reactive nitrogen sources.
3
2
The computed convex hull phase diagram depicting the 0 K and finite temperature
thermochemical stability of Mn N illustrates the challenge towards formation of Mn N under
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standard synthesis conditions (Figure 6). The free energy of formation falls within the ~35
meV/atom MAE uncertainty of the density functional calculations in resolving the on or near
3
4
hull stability of Mn N at 0 K, and it remains within this range of uncertainty for all finite
2
3
temperatures evaluated. Note that neglecting zero point energies and heat capacity contributions
3
4
to the enthalpy introduces only a modest source of error, 7 meV/atom on average, and that the
MAE of the finite temperature predictions using the statistical-descriptor for evaluating free
energies is of similar magnitude as the uncertainty in the DFT methods, ~40 meV/atom.35 From
Figure 6, we can see that if the Mn N phase decomposes to release N as an expected kinetic
3
2
2
product in vacuum, Mn N should be observed, as consistent with our observations for reactions
2
performed above 400 °C (Figure S4).
The role of controlled interdiffusion is substantiated by the observation of the solid-
2+
solution behavior of Mg Mn Cl . MgCl and MnCl are isostructural and the ionic size of Mg
x
1–x
2
2
2
2
+
and Mn are highly similar, thus the solubility (enthalpy of mixing) between these two
compounds is high (low) and they are thermodynamically miscible at the moderate
temperatures employed in our metathesis synthesis reactions.37,38 Rietveld analysis of PXRD
data shown in Figure 2 permits extraction of the unit cell parameters and metal site occupancy of
the (Mg/Mn)Cl phases, which generally agrees with the data collected in situ (summarized in
2
Figure S5 and Figure S6). The reaction product, Mg Mn Cl converges to MgCl (a = 3.636 Å
x
1–x
2
2
39
and c = 17.666 Å) as temperature increases. Given the X-ray scattering contrast between Mg
2
+
(Z = 12) and Mn (Z = 25), the refined occupancy of Mg in the salt phase also increases as a
2
+
function of temperature, converging to complete Mg occupancy as a function of temperature
and time with both precursors. Effectively, the salt solution affords the ions in the reaction
mixture a transient place for diffusion to occur.
Analysis of the in situ PXRD data shows that the MgCl forms faster than Mn N can be
2
3
2
detected. This is reflected in the different slopes of Figure 4a during the isothermal dwell at 400
C. This transient stoichiometric imbalance strongly hints at the presence of an amorphous
°
phase generated in this low-temperature reaction. As a control, a known mass fraction of
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crystalline Si (< 2 wt% amorphous component) was added into the mixture of reaction products
after the reaction was completed but before collecting PXRD data. Quantitative phase analysis
relative to the known amount of Si reveals the presence of an amorphous component in the
mixture (Figure S3). A comparison with the initial composition suggests that the amorphous
0
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component is predominantly composed of MgCl . The formation of an amorphous precursor
2
during a kinetically-controlled reaction has previously been observed by us in solid-state
1
2
metathesis reactions and warrants further future study, as it may provide access to the most
metastable polymorphs that can be synthesized.40 We do not detect the exact intermediate
containing the nitrogen, though we infer that it must contain Mn and N, otherwise the nitrogen
would escape the solid product as N (g). We do not see evidence for solid-solution behavior as
2
Mg Mn N ; therefore, we must speculate the Mn N forms by itself a second phase.
3
–x
x
2
3
2
Solid-solution behavior is not observed between LiCl and MnCl or between CaCl and
2
2
MnCl . In those cases, the cation size differences are likely large enough to increase mixing
2
enthalpies, and the phases have distinct structures; such a structural discontinuity precludes facile
pathways for itinerant ions. This factor may contribute to the observation of nitrogen-poor
Mn N phase formation in the corresponding reactions, rather than stoichiometric reaction.
4
Without a pathway for ion intercalation, it is possible that random assortment of ions within the
reaction mixture occurs, and the formation of nitrogen gas can readily take place. Upon
formation of N , the chemical activity of nitrogen is reduced to ineffective levels.
2
Figure 7. Calculated reaction free energies of nitride metathesis reactions (PR denotes the
precursor for each reaction, full reactions listed in Table SIII). Mn N formation is
3
2
thermodynamically favored over decomposition to subnitride kinetic products (i.e., Mn N
3
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
1.5 Mn N + 0.25 N ) below 900 K. Above 1000 K, decomposition to the elements is
2
2
favored (not shown). Reaction of MnCl with Mg-based precursors produces less exergonic
2
reactions than with Ca- and Li-based precursors. The uncertainty in the computational
precision of resolving the (meta-)stability of Mn N is propagated into the reaction free energy
3
2
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calculations and indicated by the shaded regions.
Thermochemistry provides a counterintuitive perspective on the experimental
observations, in that the least exergonic reactions are those which yield the targeted phase. Free
energies of reaction derived from first-principles calculations (Figure 7), illustrate that the
Mg NCl and Mg N sourced reactions are about half as exergonic as the analogous Ca NCl and
2
3
2
2
Ca N reactions, as loosely attributable to the lower ΔG (T) of the CaCl relative to MgCl , even
3
2
f
2
2
at typical synthesis conditions (T = 600-900 K). One possible explanation is that the formation
of the respective salts in these reactions generate enough heat locally that even if Mn N forms, it
3
2
will decompose into the reduced nitrides, metal, and nitrogen gas, as previously observed.8
4
1
Crystal Orbital Hamilton Population (COHP) calculations show that Mg N is more covalent
3
2
than Ca N (Figure S7), which may also kinetically inhibit N formation by having both a higher
3
2
2
Mg-N bond dissociation energy and bond directionality, inhibiting off-stoichiometry from the
liberation and transport of ions allowing the Mg N to produce the targeted phase while Ca N
2
3
2
3
produces subnitrides. Alternatively, for highly exergonic reactions (e.g., with Ca N , Ca NCl,
3
2
2
and Li N), the systems appear to form compounds that would produce the fastest change in free
3
energy (ΔG/Δt), as observed in thin film metallurgical junctions;42,43 the rapid formation of N (g)
2
as a kinetic product does not require a concerted nucleation and growth process. This may be
why Mn N is is observed instead of forming a more thermodynamically favorable MnN phase.
2
However, this rapid free energy change refers to the local reaction zone, as rapid heating of 2
Mg NCl + 3 MnCl still produces Mn N (Figure S8).
2
2
3
2
CONCLUSION
Manganese nitride (Mn N ) is formed stoichiometrically via a kinetically-controlled
3
2
solid-state metathesis reaction between 2 Mg NCl and 3 MnCl and between Mg N and 3
2
2
3
2
MnCl . The reaction proceeds without loss of nitrogen and yields crystalline products. The
2
-1
controlled reaction is afforded by a low heat of reaction (< 400 kJ mol ) and the solid solution
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between the reactants and products (MgCl and MnCl ) enables the reaction to proceed at a
2
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relatively low temperature. Free energy calculations suggest Mn N is thermodynamically stable
3
2
up to 900 K, although experiments show N formation competes as the reaction temperature
2
increases above ~380 °C. Control reactions using other nitride binaries reveal that a small free
energy change in time (ΔG/Δt) is required in synthesizing materials that decompose at elevated
temperatures, such as Mn N . Using precursors with a reasonably high nitrogen bond
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dissociation energy also appears to prevent loss of nitrogen to N during the reaction (e.g.,
2
Mg N vs. Ca N ). The use of kinetically-controlled reactions to prescribe synthesis reactions
3
2
3
2
for nominally refractory solids (e.g., nitrides) will continue to increase the diversity of nitride-
based materials available for study.
ACKNOWLEDGMENTS
Work at Colorado State University was supported by the National Science Foundation (DMR-
1
653863). JRN and AMH acknowledge partial support for collaborative work from the Research
Corporation for Science Advancement through a Scialog® Team Award. JRN acknowledges
partial support from a Sloan Research Fellowship. NRS, CJB and AMH also acknowledge
support in part for this work from the “Center for the Next Generation of Materials by Design”,
an Energy Frontier Research Center (EFRC) funded by U.S. Department of Energy (DOE),
Office of Science, Basic Energy Sciences under Contract No. DE-AC36-08GO28308 to NREL.
This research used resources of the Advanced Photon Source, a U.S. Department of Energy
(DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne
National Laboratory under Contract No. DE-AC02-06CH11357. CLR and PKT acknowledge
assistance Dr. Wenqian Xu and Dr. Andrey Yakovenko (17-BM-B, APS).
SUPPORTING INFORMATION
Supporting information is available free of charge via the internet at http://pubs.acs.org. This
content provides additional details about:

Products from Mg N + 3 MnCl and 2 Mg NCl + 3 MnCl determined by ex situ powder
3 2 2 2 2
X-ray diffraction at various temperatures and times, as well as variations in the
crystallographic parameters;

An expanded view of in situ powder X-ray diffraction data;
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
Results from DFT calculations (tables of free energies and Crystal Orbital Hamilton
Population calculations).
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