## 2 The Interstitialcy Theory  
The Interstitialcy theory is essentially based on the experimental investigations of the effect of low-temperature (4 K), low-dose soft neutron irradiation on the elastic moduli of single crystal copper carried out by [PERSON]'s group in the 1970s [17; 18]. The irradiation leads to the formation of isolated Frenkel pairs (vacancy+interstitial) remaining mostly immobile in the liquid helium temperature range. A careful analysis of the experimental data led to the conclusion that the formation of interstitial defects results in a strong decrease of the shear modulus (diaelastic effect) \(C_{44}\) according to \(\Delta C_{44}/C_{44}=-B_{i}c_{i}\), where \(c_{i}\) is the interstitial concentration and \(B_{i}\approx 30\) is the shear sensitivity. For the vacancies, the shear sensitivity is by an order of magnitude smaller, \(B_{v}\approx 2\). The effect of point defects on the bulk modulus is also small, comparable to the diaelastic effect produced by the vacancies. At the same time, similar results were obtained upon electron irradiation of single crystal aluminum [19]. It was argued that a strong diaelastic effect can be observed only if the interstitials do not make up an octahedral configuration, as considered before the 1970s (red circle in Figure 1a), but form a dumbbell (split) structure, which is characterized by the two atoms trying to occupy the same lattice cite (red circles in Figure 1b). To underline the difference between these defects, the latter defect is also called an interstitialcy [20]. It is of major importance that the octahedral interstitial is a spherically symmetrical defect (similarly to the vacancy) while the dumbbell interstitial is clearly anisotropic and, thus, constitutes an elastic dipole strongly interacting with the external shear stress [19; 21]. It is this feature of the defect, which leads to a strong diaelastic effect. It is now generally accepted that dumbbell intertitials exist in all main lattices and constitute the defect state with the lowest formation enthalpy [22; 23; 24; 25; 26]. An important consequence of the interstitial dumbbell structure consists in the appearance of the low-frequencies in the vibrational spectrum, which are by a few times smaller than the Debye frequency [28]. This in turn leads to the high formation entropy of the defect (for copper, \(S_{f}\approx(5-10)\)\(k_{B}\) according to the [PERSON]'s estimate [14]) and, respectively, to a decrease of the Gibbs formation free energy at high temperatures. The extrapolation of the elastic moduli of irradiated crystals towards high defect concentration in the experiments [17; 18] showed that \(C_{44}\to 0\) at \(c_{i}\approx 0.02\div 0.03\) providing a guess that such big defect concentration should lead to the crystal\(\rightarrow\)liquid transition, because the liquid is characterized by a vanishing (or very small) shear modulus [29; 30]. This allowed [PERSON] to suggest that melting of metals should be related to the rapid thermoactivated generation  
Figure 1: Octahedral (**a**) and dumbbell (**b**) interstitial defects in a computer model of a face-centered cubic lattice [27]. All of the dumbbell atoms (marked by red circles) are characterized by \(<0,2,8,0>\) Voronoi indexes. With permission from JETP Letters, 2019.  
of dumbbell interstitials. Another important point realized by [PERSON] was the understanding that the defect formation enthalpy is proportional to the unrelaxed shear modulus \(G\), in line with earlier investigations [31; 32]. The above hypotheses led [PERSON] to the formulation of the interstitialcy theory, which includes melting as an integral part [13; 14] (see also a discussion given below).